EMBRYOLOGY

it remains here to deal with the more recent discoveries as to the nature and meaning of the developmental processes. the cell—we take for granted (see cytolocy) a general acquaintance with the structure of the bodies of adult animals. it is now a matter of universal agreement to conceive the active living parts of these bodies, which are included under the general name protoplasm, as built up of a series of units termed cells, each normally containing a single nucleus and separated from one another by quasi-solid membranes termed cell-walls. the doubts as to the validity of the concept of the cell, which were raised in the later years of the 19th century, have not been sustained by later discoveries. a more refined technique has enabled us to demonstrate a cell-wall in cases where it was 970 supposed to be absent; and where it really is absent, as for instance in the ectoderm of the nematode worms, it has been proved that this is a secondary state of affairs, due to the degeneration of a well-developed layer of cells, which in younger stages of the life- history are clearly and sharply delimited from each other. in many, perhaps in most, cases the cell-walls are perforated so that adjacent cells are connected by bars of protoplasm, but this circumstance in no way invalidates the idea of the cell as the unit of structure. scope of embryology.—the lowest grade of animals, termed the protozoa (see protozodlocy), do not exhibit cellular structure. either their bodies are so small that they possess only one nucleus, and may be regarded as free-living cells; or they contain more than one nucleus and attain a greater size, and then their protoplasm is not divided into compartments in accordance with the distribution of these nuclei. some of the largest of the protozoa, such as the extinct genus nummuilites, were disk-like in form and attained a size of an inch in diameter; the bodies of these animals were divided into thousands of compartments by calcareous septa. to judge from what we know of the structure of their nearest living representatives they must have possessed numerous nuclei; but these nuclei were not dis- tributed in accordance with the divisions of the protoplasm. some compartments contained several nuclei, some one nucleus only and many none; so that true cell-structure was absent. in other cases the protozoen may be described as a colony of small uninucleate forms, connected together either by strings of proto- plasm or by stalks springing from a common base. but all these more complex protozoa are distinguished from the true higher animals or metazoa by the fact that when reproduction takes place the whole body of the parent breaks up into germs, each containing a single nucleus, whereas in true metazoa small portions only of the parent’s body are set aside for reproductive purposes; in other words, in the metazoa there is a persistent ‘‘ soma " or body distinct from the germ-cells. now of course the development of the protozoa ought to form part of the subject matter of embryology, but in the case of the smaller species it is exceedingly difficult to say which stage corresponds to the adult condition of metazoa, since repro- duction by the division of the mother’s body into two can take place at various periods in the life-cycle; therefore purely as a matter of convenience it is customary to confine the subject matter of embryology to the study of the life-histories of the higher animals which exhibit definite cellular structure, in a word, to the metazoa. metazoa.—if we now examine the development of the metazoa we find a few cases where, side by side with other methods, reproduction by fission, that is by the division of the mother’s body, does actually take place. thus in the marine annelid procerastes described by allen! the mother worm breaks up into groups of one, two or three segments and each of these groups regenerates the missing parts and thus reconstitutes a new worm. in much more numerous cases an out- growth of the mother’s body, termed a “ bud,” is produced. the bud may consist from the beginning of one or of several tissues, and is slowly moulded into the likeness of the parent and when fully grown separates from it, or in the case of a colonial animal remains connected with it and helps to build up a compound organism. such compound creatures are found amongst the sponges, the coelenterata, the polyzoa and the ascidians, the last-named group being de- generate allies of the vertebrata. the germ cells.—the laws of bud-development have not been as clearly clucidated as those of the germ-cells. development by germ-cells is universal amongst the metazoa; and in all but two phyla the form in which they appear is remarkably constant. they are of two kinds, viz., male and female, and are normally incapable of development unless they have previously united in pairs to form what are called “ zygotes ” (gr. ¢veyv, a yoke). the male cell or spermatozoien consists of a head which is a con- densed nucleus made up of a compact mass of chromatin, and a tail which is a vibratile filament. amongst the nematode worms, how- ever, the male cells are devoid of filaments and appear under the form of small amoeboid cells, whilst amongst the higher crustacea (7.e., the shrimps, lobsters and crabs) the tail is replaced by a peculiar open vesicle lined with chitin, which under certain circumstances absorbs water and becomes turned inside out, thus propelling the head forwards and bringing about the union of the two germ-cells. the female cell or ovum (egg) is typically rounded and motionless but it is of very different sizes in different species of animals. these differences in size depend entirely on the varying amounts of food- 1. j. allen, ‘’ an autotomy and regeneration in the syllid worm procerastes,"” phil. trans. roy. soc. lond., series b, vol. 211 (1921). embryology yolk (i.e, reserve material) deposited in the cytoplasm (extra- nuclear protoplasm). the food-yolk in turn differs in amount ac- cording to the extent to which the young organism must grow be- fore it can obtain nourishment for itself. thus the human egg is only about 0-1 mm. in diameter, since at a very early period of its development it becomes attached to the wall of the womb and sub- sequently draws all its nourishment from that source. the egg of the ostrich on the contrary is one of the largest known, being about 15 cm. in diameter, since it has to provide all the food necessary to build up a good-sized chick. eggs which have a very small amount of yolk and in which this is evenly distributed throughout the cytoplasm are termed “alecithal ”’: such are the eggs of hydrozoa, echinodermata, brachiopoda and of amphioxus and mammalia amongst vertebrata. eggs in which the yolk is concentrated at one pole of the egg are termed “ telolecithal ”’: this pole is termed the ‘‘ vegetative pole,’’ whilst the opposite pole where the bulk of the cytoplasm is concentrated and where the polar bodies (see below) are given off is termed the ‘anima! pole.” the eggs of most annelida and mollusca and of pisces, amphibia, reptilia and aves amongst vertebrata are telolecithal. eggs in which the yolk is massed in the central part of the egg and is sur- rounded by a layer of cytoplasm almost free from yolk are termed “ centrolecithal.” in this class are the eggs of nearly all arthropoda. both types of germ-cell before attaining maturity undergo two ripening (maturation) divisions, so that in each case four daughter cells are produced. whereas in the case of the male germ-cell all four daughters become fully formed ‘“ spermatozoa,” in the case of the female germ-cell only one daughter is converted into the ripe egg; the remaining three are small vestigial cells destined to perish, which are termed “ polar bodiecs.”’ during the maturation divisions the number of chromosomes in the nuclei of both male and female germ-cells is reduced by one-half (see cytology). when the spermatozoen enters the egg, the head, which is a con- densed nucleus, swells up and assumes the ordinary nuclear structure and is termed the ‘“‘ male pronucleus '"; behind it is situated a very active centrosome which produces a series of radiating rays termed the “ spermaster.’’ the nucleus of the ripe egg is termed the ‘' female pronucleus.’’ the male pronucleus approaches the female pronucleus, and becomes closely apposed to it; the spermaster then fades out, the centrosome which gave rise to it becomes divided into two which become connected by a so-called ‘‘ spindle.’?” new asters are then formed from the daughter centrosome at the poles of the spindle; this spindle initiates the development of the egg by bringing about the first division of the combined male and female pronuclei and of the fertilised egg (zygote) itself. the tail of the spermatozoen is either left outside when the head penetrates the egg, or if it pene- trates the cytoplasm it degenerates there; its remnants can some- times be detected in one cell of the embryo, up till the stage of 32 cells has been attained, but it takes no part in cell-division and no portion of it is transmitted to any other cell, the conclusion being that it plays no part in the transmission of hereditary qualities. the nucleus of the zygote, as we have just seen, ee double the number of chromosomes which are present in the nucleus of the ripe egg but half of these are of male origin. every nucleus of the de- veloping embryo therefore inherits from the zygote nucleus an equal number of male and female chromosomes, so that the body of the embryo has with justice been likened to a piece of cloth of which the warp is paternal and the woof maternal. parthenogenests.—in the earlier article it was pointed out that the unfertilised egg could be induced to develop by a variety of agencies varying from the addition of a small quantity of butyric acid to the sea-water in which it is placed, followed by exposure to the action of hypertonic (i.¢., over-salted) sea-water in the case of echinoderm eggs, to the prick of a pin in the case of the eggs of amphibia. this is termed artificial parthenogenesis. in the case of the eggs of sea-urchins parthenogenesis has been minutely studied by loeb? who has put forward various thconies' ag to action of the agents which he employed. he imagined thaf the action of the butyric acid was to start cytolysis, one result of which was the formation of a definite egg membrane, but which if unchecked destroyed the egg, which became resolved into a mass of globules. the exposure to hypertonic sca-water was supposed to arrest this injurious action. this explanation was obviously not applicable to the parthenogenesis of the frog’s egg. the whole subject has been attacked from a new point of view by herlant® and brachet! who have pointed out that the agent em- ployed to provoke parthenogenesis does not exercise a specific chem- ical action on the egg but merely acts as a stimulus to which the egg as a living organism responds. whether butyric acid or a needle be employed the response is the same; the egg ‘“‘ wakes up” so to 2 j. loeb. numerous papers summarised in his book, die chemische tentwicklung des tierischen fetes (1909). > m. herlant, ‘le mecanisme de la parthenogenese experimen- tale,” bull. scientifique de la france et de la belgique, 7th series, vol. 50 (1917). * a. brachet, “ l’oeuf et les facteurs de l’'ontogenese,"’ encyclo- pedie scientifique (1916). embryology speak, the nucleus emits something which acts as a centrosome and from this is developed a great series of radiating rays traversing the cytoplasm, a huge ‘‘ monaster’’ in fact. the chromatin of the nucleus becomes resolved into chromosomes which are split longi- tudinally and which become adherent to the rays of the “* monaster.” in the case of the egg of the sea-urchin it is only extremely rarely that the monaster becomes changed into an ordinary mitotic spindle by the division of the centrosome. in most cases after persisting for about an hour the monaster disappears; the nucleus returns to the resting condition and then after a short interval it passes through the same phases, a monaster being again formed. after this process has been repeated about six times over a period lasting twelve hours the egg dies and cytolysis supervenes. if, however, after the egg has been exposed to the action of the butyric acid and then washed in sea-water, it is placed in hypertonic sea-water, and then after a limited period of immersion in this fluid replaced in ordinary sea-water, additional asters are formed in the cytoplasm. when the egg forms a monaster this becomes connected with these other asters by longitudinal fibres so as to form a complex spindle. by properly choosing the period of immersion in hyper- tonic water it is possible to arrange that only one additional aster should be formed; the centrosome of this aster then joins with that of the monaster to form a normal mitotic spindle on to which the egg chromosomes migrate; a regular division of the nucleus follows and thereafter a division of the whole egg into two cells, and so parthenogenetic development is initiated. these two cells are generally very unequal in size, whereas in normal development the first division of the egg results in the pro- duction of two cells of exactly the same size. this inequality results from the fact that the aster induced by the action of the hypertonic water is small in comparison with the size of the monaster produced by the butyric acid. the size of the cell produced at the division of the egg is therefore determined by the size of the aster at one pole of the mitotic spindle. the aster is a framework of semi-rigid gela- tinised protoplasm, which becomes continuous with the viscous proto- plasm forming the outer layer of the cytoplasm of the cell. the course of events in the frog's egg is fundamentally similar to the process which we have just described, although there are differ- ences in detail. a prick with a sterilised needle induces the formation of a huge monaster, the centrosome of which then divides into two, between which a short mitotic spindle is formed on to which the chromosomes of the egg migrate. since, however, the length of the spindle stands in relation to the number of chromosomes in the nucleus and as these chromosomes are only present in half the num- ber found in the nucleus of the fertilised egg, the spindle which is formed is only four-fifths of the length of the first spindle formed in the fertilised egg. the length of spindle in turn determines the length of the astral rays from its poles, and if these are too short to reach the periphery of the egg the spindle is unable to bring about the division of the egg into two cells. this is the case with the spindle formed in the parthenogenetic egg, and although abortive and transitory furrows on the egg’s surface are formed no division into cells results; the nucleus, it is true, divides and a multiplication of nuclei follows in which the numerous short spindles formed in- terfere with one another and make orderly development impossible, and so after a short time the egg dies. if, however, the needle be “ infected ” by being dipped into frog's blood before being used to prick the egg, then the foreign substance thus introduced produces additional asters in the cytoplasm just as did the hypertonic water in the sea-urchin’s egg. these asters have a tendency, as their rays develop, to repel one another, and they push the mitotic spindle developed around the egg nucleus over to one side. if this side happens to be the side of the egg at which the cytoplasm is concentrated, then the spindle is able to start the forma- tion of a furrow which cuts right through the egg and divides it into two cells, and so parthenogenetic development is begun. we sce then that the difficulty of initiating parthenogenesis depends on two factors, viz.: (1) the quiescent condition of the egg and (2) the small amount of chromatin present in the nucleus. if we choose the unripe eggs of the sea-urchin as the subjects of our experiment, then it is sometimes possible to induce them to develop by the use of one reagent alone, such as hypertonic sea-water; since in these eggs the “reducing " division of the nucleus has not occurred and the chro- matin is consequently present in undiminished quantity. parthenogenctic development is closely related to the problem of heterogeneous fertilisation. it has been shown that under certain circumstances it is possible to fertilise the eggs of the sea- urchin with the sperm of creatures so diverse in zoological affinity as an annelid worm (chaetopterus) and the sea-mussel (af ytilus). in the first case the male and female pronuclci fuse, but the male chromatin falls out of the zygote nucleus before the first division takes place. in the second case the male pronucleus refuses to enter into union with the female pronucleus at all, but the spermaster brings about the division of the egg. when the eggs of the sea-urchin (echinus) are fertilised with the sperm of the heart-urchin (echinocardium), in the vast majority of cases 971 cytolysis results exactly as it does after the exposure of the echinus cggs to the action of butyric acid, but insome few cases the egg develops and produces a hybrid. we conclude that in most cases the sperm of echinocardium is so alien to the cytoplasm of the egg of echinus that it is not even able to bring about the formation of a spermaster. under certain circumstances (slight staleness of the egg, excess of sperm, etc.) more than one spermatozoin may enter the egg. in large eggs such as those of cephalopods, reptiles and birds, this seems to be a normal occurrence; only one of these nuclei unites with the female pronucleus and forms the zygote nucleus from which begins the cell- division which initiates development; but the other spermatozoa also form centres for cell-division, which gives rise to the so-called free cells which are characteristic of these eggs. these free cells are gradually crushed out and destroyed by the developing cells pro- duced by the activity of the zygote nucleus. brachet, however, has shown! that when the frog’s egg is entered by spermatozoa in moderate numbers, whereas only one fuses with the female pronucleus, the others form centres for the formation of cells which are built up into the body of the embryo. as these sperm-heads, however, contain only half the quantity of chromatin contained in the zygote nucleus, the cells to which they give rise are markedly smaller than those which contain nuclei descended from the zygote nucleus, and so it is possible to distinguish in the growing tadpole the regions which contain cells which have nuclei derived from the zygote nucleus from those which contain cells having nuclei derived from the supernumerary spermatozoa. brachet's observations prove in the clearest manner that the differentiation of organs in the frog’s egg is due to the differentiation of regions in the cytoplasm and not to the differentiation of the nuclei produced by the division of the zygote nucleus as weismann? had supposed, for some of these nuclei can be replaced by sperm- nuclei, each of which carries in it the potentiality of producing the whole organism—not a mere region of it—and yet no dislocation of development results. the entry of two or more spermatozoa into smalleeggs such as those of the sea-urchin usually produces abnormal development followed by early death. the reason is that the centrosomes which are carried into the egg by these spermatozoa are so near each other that instead of leading to the formation of separate spindles they give rise to three- (triaster) or four-poled (tetraster) spindles along which the chromosomes are arranged in an irregular manner. this causes the formation of abnormal nuclei incapable of properly ful- filling their functions and the embryo dies. development of the egg.—lif we now turn to consider the norma] development of the egg we find that this can be divided into three stages which in primitive forms are sharply delimited, but which in more modified forms tend to overlap one another. these stages are (1) segmentation, or the division of the egg into a number of indifferent cells or blastomeres; (2) the formation of the so-called germ-layers, 7.¢e., the differentiation of the blas- tomeres into the primitive organs, viz.: (a) the ectoderm (or epiblast) which is the primitive skin, (6) the endoderm (or hy- poblast) which is the primitive lining of the gut, and (c) the mesoderm (or mesoblast) which is the primitive peritoneum or lining of the body-cavity; (3) organogeny, 7.e., the formation of the separate organs of the body, such as brain, liver, kidneys, ctc., from the germ-layers. segmentation of the fgg.—considering first the process of segmen- tation, we find, as balfour? pointed out long ago, that the effect of the accumulation of yolk in the egg is to impede cell-division. it acts exactly as if it were a dilutant of the cytoplasm in lowering sur- face tension. cell-division is accompanied by partial gelatinisation as is obvious from the way each daughter cell rounds itself off from its sister. this is particularly evident in the segmentation of alecithal eggs, for in them, in the early stages of segmentation, all the blas- tomeres divide simultaneously, and just after each period of division these take on the appearance of a pile of balls only touching each other in points; whereas during the interval between two such periods the gelatinisation diminishes and the blastomeres become flattened out against cach other. in all alecithal and telolecithal eggs there is a pole (see ahhove) from which the polar bodies are given off which is termed the animal pole of the egg. this pole is the region of the egg which contains least yolk; here cell-division is most rapid and the smallest blastomeres are produced, whereas as we pass towards the vegetative pole of the egg, where the yolk is concentrated, the blastomeres become fewer and larger. when the yolk is very much increased in amount, the nuclei pro- duced by the division of the zygote nucleus are unable to produce 1 brachet, joc. cit. * : 2a. weismann, the germ-plasm, a theory of heredity (1900). 3 i". m. balfour, comparative embryology, vol. 1, p. 95 (1880). 972 an amount of gelatinisation sufficient to divide the cytoplasm, and so we get a multiplication of nuclei without the formation of blastomeres. when this happens segmentation is confined to the animal pole of the egg and results in the formation of a thin disk of blastomeres termed the “ blastoderm,” resting on an unsegmented “ yolk.” such eggs (for instance the hen’s egg) are termed ‘‘ mero- blastic (gr. wepos, a part) in contradistinction to eggs, like those of oe frog, which are completely divided and are termed ‘ holo- astic.’ in centrolecithal eggs, like those of the cray-fish, the egg appears to be completely divided into cells, but although division may at first be complete, when gelatinisation diminishes the inner yolky ends of the blastomeres flow together so as to form a common inner yolky mass. such eggs are said to exhibit superficial segmentation, later, the outer protoplasmic ends of these incomplete blastomeres become completely cut off, so as to forma skin of cells of blastoderm surrounding a central “ yolk.”’ a still further modification of this type is found in the eggs of insects in which the yolk is so abundant as to prevent all segmentation. the zygote nucleus alone divides and gives rise to daughter nuclei each surrounded by an island of protoplasm; these are at first dispersed throughout the ‘‘ yolk,’ but they gradually migrate to the surface and here form a blastoderm. in primitive alecithal eggs segmentation results in the formation of a hollow ball of cells one layer thick. this ball is termed the ‘blastula ” and its cavity the ‘“‘ blastocoele,”’ ‘ segmentation- cavity " or “ primary body-cavity.” the formation of the blastula marks the accomplishment of an important step in development. although typically formed only in alecithal eggs, it appears in a modified form in telolecithal eggs, even in those in which there is so much yolk that they have meroblastic segmentation. thus in the case of the frog the blastula is a hollow ball of which the roof is two cells thick and the floor is many cells.thick, whilst in the case of the pigeon the blastula is represented by a stage in which the blasto- derm is one layer thick and forms the roof of the slit-like segmenta- tion cavity, whilst the immense mass of the unsegmented yolk forms the floor of this cavity. this floor, in its uppermost layer, contains a few nuclei which are representatives of the cells which should constitute the vegetative pole of the blastula but these nuclei are utterly unable to cut the yolk up into cells. formation of germ layers.—as soon as the blastula stage has been attained, the “ formation of layers ’ begins. the cells at the vegeta- tive pole become turned inwards, forming a tube-like structure which projects into the blastocoele and partially obliterates it. this tube is the primitive gut or “archenteron”’ and the cells forming it are termed ‘ endoderm,” whereas the cells forming the outer wall of the blastula give rise to the primitive skin and are termed “‘ ecto- derm.”’ recently, von ubisch' has introduced an ingenious method of staining certain blastomeres during life. by the aid of this method he has been enabled to determine definitely not only the fate of individual blastomeres in normal development but also their powers of development (prospective potencies) when separated from their fellows. when in the 8-cell stage of the sea-urchin’s egg the four upper cclis are isolated, they will produce a blastula, but this blas- tula will neither produce ‘ mesenchyme ”’ (that is, wandering cells which migrate into the blastocoele) nor an archenteron, but the lower four cells will give rise to a blastula which will produce mesenchyme and an archenteron and which eventually grows into a perfect larva of reduced size. in normal development these four lower cells are entirely used up_in the formation of archen- teron and mesenchyme; whence von ubisch concludes that prospec- tive ectoderm is unchangeable but that prospective endoderm retains all the primitive potencies of the egg and can be changed into ectoderm. when the archenteron has been formed, the developing egg has assumed the shape of a double-walled cup, the opening into which is termed the “ blastopore.” this stage is clearly and sharply marked in the development of almost all eggs in which the yolk is small in amount, and it can be recognised in an obscured and altered form in the development of large yolky eggs. it is of equal impor- tance with the blastula stage, and it is termed the ‘ gastrula.”’ the primary body-cavity has now become reduced to the slit in- tervening between the wall of the archenteron and the outer wall of the gastrula and this slit becomes largely filled up by the develop- ment of the third germ layer, the ‘‘ mesoderm.”’ we have defined this layer as the primitive peritoneum or lining of the body-cavity, but the body-cavity now indicated is termed the “‘ coelom”’ or ** secondary body-cavity ”’ in order to distinguish it from the primary body-cavity. in the eggs of primitive animals, where the yolk is small in amount, the coelom is always formed as a series of pouch- like outgrowths of the archenteron which become cut off from this tube. it follows that the mesoderm is differentiated from the pri- mary endoderm. driesch? has shown that if the front half of the gastrula of the starfish which includes the apex of the archenteron be cut off, the hinder half will heal up and will form a perfect larva, forming, of course, the coelom in the normal way. hi, however, this 1von ubisch, ‘ entwicklungsphysiologische studien an seeige!- keimen,”’ zeit. fiir wiss. zoologte, vol. 124 (1935). 2h. driesch, “ zur analysis der potenzen embryonalen organ- zellen, arch. f. entwicklungsmechantk, vol. 2 (1896). embryology operation he performed after a swelling of the tip of the archenteron (the first rudiment of the coelom) has appeared, then, although the hinder half will heal up and forma larva, it never forms a coclom. driesch concludes from this experiment that at first all parts of the archenteric wall have the power of giving rise to a coelom, that is of forming mesoderm, but that later a definite portion of this wall be- comes sect aside as the rudiment of the coelom and that then the rest of it becomes the definitive endederm devoid of this coclom- forming power. in echinodermata the coelom arises as a single pouch from the apex of the archenteron; in primitive vertebrata it originates as five pouches of which one is apical and four are paired and lateral; in chaetognatha and brachiopoda as a lateral pair of pouches. the remnant of the primary body-cavity becomes almost filled up with cells budded from the wall of the coclom which are termed “‘ mesenchyme.”” the primary ‘‘ mesenchyme ” of the devel- oping echinoderm, however, is formed from that portion of the blastular wall which will later be converted into the archenteron. after the archenteron has been formed the production of mesenchyme continucs but is now confined to its apex, 7.e., that portion which will be converted into the coelom. these cells may become joined to one another by their processes and thus constitute a network which be- comes converted into connective tissue by the secretion of fibres; or they may remain separate from one another, and then they become developed into blood and lymph cells, the remnants of the primary body-cavity constituting the blood-spaces. in the coelenterata, in which no coclom is formed, similar cells are budded from both ectoderm and endoderm; in annelida and mollusca, in addition to the mesenchyme given off the coelomic wall, some is likewise budded from the ectoderm, and to this the name ‘‘ mesectoderm” has been given, in vertebrata the most recent research indicates that no mesenchyme is given off from the ectoderm. organogeny.—turning now to the third stage of development, viz., the formation of special organs, we find that from the ectoderm are derived the central nervous system and the sense organs, and also the lining of the mouth-cavity and of the terminal portion of the alimentary canal near the anus. the endoderm gives rise to the middle portion of the gut and to the glands which are developed from it, and in vertebrata to the primitive elastic axis of the backbone or ‘“ notochord.”’ from the mesoderm arise the majority of the muscles, the connective tissue and, in vertebrata and echinodermata, the internal calcareous skeleton which is derived from the connective tissue. the mesoderm also gives rise to the genital organs and their ducts in all metozoa above the rank of coclenterata, and in mollusca and vertebrata to the kidney tubules. now we have pointed out that, in telolecithal eggs, segmentation proceeds most rapidly at the animal pole; here the second stage of development rapidly supervenes, and the archenteron is begun before segmentation is even initiated at the vegetative pole. in meroblastic eggs the upper pole of the egg may become converted into an embryo in which all the important organs of the adult are mapped out before the lower pole is even invested with cells. finally in amniota (reptiles, birds and mammals) the lower pole of the egg, after all the yolk has been absorbed from it, is torn from the rest of the embryo at birth and cast off as a useless embryonic membrane, in the earlier article doubts were expressed whether the primitive germ layers corresponded to one another in different eggs; in a word, whether the same name had not been given to different things. the result of the labours of embryologists has been to establish the uni- versal homology of the germ layers on an ever firmer basis. if, for instance, we define the mesoderm as the wall of the coelom then it is found that this organ originates in one of two ways, viz., either as a pouch or a mass of cells. the pouch (recognisable in chaetognatha, brachiopoda, echinodermata, enteropneusta and the lowest vertebrata) quite clearly originates as an outgrowth from the endoderm; the mass of cells can be traced back to its source in one large cell, the mother mesoderm-cell. this cell, as was first shown by shearer? in the annelid f7ydrotdes and by conklin‘ in the mollusc crepidula, originally forms part of the wall of the archen- teron, and its ejection from this wall is evidently a modification of the more primitive method of coelom-formation by the outgrowth of a gut-pouch. meissenheimer’ and harms* stated that in certain mollusca the coelomic pouches originated from cells budded from the ectoderm. later workers have shown that in these very mollusca the mother mesoderm cell gives rise to the pericardium which is representative of the coelom in these animals. the so-called ‘‘mesec- toderm "’ of annelida and mollusca gives rise to some superficial muscles, but to confound this with the coclomic wall and its deriva- tives by calling both mesoderm and then to complain that the meso- derm is not an homologous structure in various groups of animals is to introduce a perfectly gratuitous confusion. the gut epithelium of certain arthropoda has been alleged to arise 3c. shearer, ‘‘ on the development and structure of the trocho- pore of hydroides,”’ quar. jour. micr. sci., vol. 13 (n.s.) (1911). 4e, g. conklin, ‘‘ the embryology of crepidula,” journal of morphology, vol. 13 (1897). ' j. meissenheimer, ‘‘ entwicklungsgeschichte von dreissensia polymorpha,”’ zettschrift f. wissenschaftliche zoologie, vol. 69 (1901). ®w. harms, ‘ postembryonale entwicklungsgeschichte der unioniden,’’ zeol. jahrbiicher (abt. fiir ontogenie), vol. 28 (1909). embryology from the ectoderm. if we examine the embryo of the most primitive land arthropoda peripatus we find a large slit-like blastopore which later became divided by a constriction into mouth and anus, it has been suggested that a portion of the gut epithelium, viz., that forming the midventral portion, is formed from ectoderm turned in round the edges of the slit. now in the primitive annelid polyvgordius wol- tereck! has described a similar slit-like blastopore and he has fol- lowed the process of its closure in great detail, describing the division of every cell involved. in this case the midventral epithelium of the gut is formed by the union of endoderm cells lying at the sides of the blastopore—whilst the ectederm cells lying in the blastoporal lips by their union reconstitute the midventral skin. doubtless a renewed investigation with a more modern technique would show that this 1s also true of peripatus. embryological research is based on a comparison of embryos of different ages with one another—not, as would be the ideal method, on a continuous observation of the progress of one and the same embryo. it follows that too large an age-difference between the embryos examined may give rise to a totally wrong conception of the process which is taking place. so is to be explained the state- ment, by heymons? that the mid-gut of the higher insects is entirely formed from ectoderm and that by watasc* who attributed a simi- lar origin to the mid-gut of the cephalopod lolige. hirschler* has shown how the first error originated, and the second has been cor- rected by faussek®; and should further statements of this kind oc- cur the strong presumption is that they also are founded on mistakes. organ-forming substances—we have arrived at the conclusion that the establishment of the validity of the germ-layer theory is one of the great achievements of embryological research, and we now turn to the question of how the differences which dis- tinguish the layers from one another are brought about. we have learned that in primitive alecithal eggs like those of echinoderms the lower portion of the blastula wall has the capacity of giving rise to a complete larva and that this capacity only becomes re- stricted when the first traces of gut-formation are visible. we have also learned that all parts of the primitive gut or archen- teron are alike in their coclom-forming powers, and that the separation of endoderm from mesoderm only becomes apparent when the first indication of the coelom appears. but this pro- gressive differentiation of the embryo might be due to a dif- ferentiation of the nuclei of various regions or of the cytoplasm or of both. we have, however, learned from the development of the polyspermic frog’s egg that there is a strong presumption that the nuclei of the embryo are alike in their nature and that the differentiation of the layers must be due to the separation of organ-forming cytoplasmic substances from one another. this conclusion is confirmed by a large number of observations on many diflerent kinds of eggs; a few of the more striking follow. hertwig® allowed frogs’ eggs to develop under pressure between glass plates and in capillary tubes. under these circumstances the divisions took place by planes normal to the pressure and flat plates and rows of cells were produced. when the pressure was removed, however, these deformed embryos recovered, multiplication of cells took place and the normal form was regained and normal develop- ment proceeded. it was casy to show that nuclei which under undis- turbed conditions would have occupied certain definite regions of the embryo had been forced into quite other regions, and yet perfectly normal embryos resulted. hertwig concludes that the nuclei could be juggled abeut like a handful of bails without affecting the forma- tion of the embryo. : in many eggs the differentiation of the layers is indicated at a far earlier period than that at which it occurs in the egg of the echinoder- mata or even of the lower vertebrata like the frog. the egg of the ascidian cynthia partita which has been studied in a detail by conklin? may be adduced as an example. this egg when it develops 1 woltereck, “ beitrag zur praktischen analyse der polygordius- entwicklung,” archi:. f. entwicklungsmechanik, vol. 18 (1903). 22. hleymons, ‘ uber die bildung der keimblitter bei den in- secten,” szisungsb. der preussischen akad, der w'tss., vol. 1 (1894). 3 watase, ‘‘ observations on the development of cephalopods,”’ studies from the biol. lab. johns ilopkins univ., baltimore, vol. 6 (1888). 4]. hirschler, ‘‘embryonalentwicklung von donacta crassipes,” zeitsch. f. wiss. zool., vol. 92 (1909). ’v. faussek, “ untersuchungen tber die entwicklung der cephalopoden,”” mitteilungen a. d. zool. station 2u neapel, vol. 14, p. 83 (1901). on ” 6q. hertwig, ‘‘ ueber den werth der ersten furchungszellen tiir die organbildung des embryos,” archiv f. mikr. anatonite, vol. 42 1893). a g. conklin, ‘‘ the orientation and cell-lineage of ascidian egg,” jour. acad. sciences, philadelphia, series 2, 13 (1905). the vol. 279 becomes converted into an elongated blastula consisting of few cells- this blastula changes into a gastrula in the typical way and, though no distinct coclomic pouches are formed, large portions of the archen- teric wall are directly converted into muscles which lie at the sides of the tail of the tadpole-like larva. in this species the nucleus of the unripe egg is, as usual, a vesicle filled with fluid (the so-called ger- minal vesicle). ‘the cytoplasm contains numerous yolk globules of a slaty-blue colour and also larger yellowish globules which are con- centrated in its superficial layer. when the maturation divisions of the nucleus occur the nuclear wall is dissolved and the fluid contents escape and forma cap of clear material at the anima! pole of the egg. when fertilisation takes place profound rearrangements of the substances in the cytoplasm are effected. the yellow globules stream downwards to mect the spermatozoen which enters at the vegetative pole, and they finally form a crescentic layer of yellow material round the lower pole of the egg. as the egg develops first into a blas- tula and then into a gastrula, and finally into the characteristic ascidian tadpole, it becomes evident that the clear substance forms the ectoderm, that the slaty-blue material is-contained in the endo- derm, whilst the yellow material is confined to the masses of mesoderm which give rise to the tail muscles. ° when the egg is in the four- cell stage the yellow material is confined to the two posterior cells; if one of these be killed the remainder of the egg will give rise to a larva with muscles on one side only of the tail. that the nuclei have nothing to do with this separation of substances is shown by what occurs at the lip of the blastopore. i[cre we find an arc of what conk- lin calls ‘‘ neurochordal ” cells. each of these has of course a single nucleus, but the cytoplasm of each consists of two zones, one clear and one slaty-blue. at the next division two daughter cells are pro- duced from each neurochorda! cell; one of these contains the clear substance and is added to the nerve plate which is a part of the ectoderm; the other is composed of the blue substance and forms part of the notochord which in cyzihia as in other vertebrata is a derivative of the endoderm. from this development we conclude that the germinal layers owe their origin to the segregation of cytoplasmic substances 1n the growing egg; that these substances assume their final arrangement under the influence of the spermatozoen which thus, on its path to meet the female pronucleus, determines the symmetry of the embryo. but it would be a mistake to conclude that in these visible yellow and blue materials we have the actual organ-forming substances themselves. the nature of these substances in the egg of cynthia has not yet been determined but similar substances occur in the eggs of many mollusca and more particularly in those of limnaea planorbis and physa. in these eggs, too, as development proceeds these substances are distributed to definite regions of the egg, conklin’ has centrifuged the eggs of these snails and produced thereby a completely abnormal distribution ‘of these substances and yet the development of the egg proceeded in a perfectly normal manner to the formation of a healthy young snail. brambell? has shown that the greyish or blu- ish material is in all probability golgi yolk and the yellow substance swollen mitochondria (see cyrology). brachet!® has shown that there are also organ-forming regions in the frog’s egg. it was for long a puzzle why competent observers like roux'! and hertwig” should differ so profoundly on the results of killing one of the first two blastomeres of the frog’s egg. roux asserted that the surviving blastomere gave rise to a half blastula which developed into a half tadpole, whilst hertwig maintained that it tended to form a normal tadpole, being only impeded in its development by the mass of deat material constituted by the other blastomcre. brachet has shown that both are right, for the plane separating the first two blastomeres need not by any means coincide with the future median plane of the embryo, but may make any angle up to a right angle with it, if it coincides with this plane by killing one blastomere roux’s result is obtained; if it is oblique the result accords with hertwig’s researches, thus the potency of each of the first two blastomeres of the frog’s egg depends entirely on the cytoplasm which it happens to include and in no way on the nucleus. brachet" has shown that the fixing of the median plane of symmetry in the frog’s tadpole, as in the ascidian tadpole, is effected by the spermatozoen. as the sperma- tozo6bn penetrates the egg in its path towards the female pronucleus, it leaves behind a trail of pigment which persists for a considerable time and can be detected at a much later period in the development of the egg. on the opposite surface of the egg to that at which the spermatozoon enters it, there is formed the so-called “ grey crescent.” this is in reality the upper lip of the blastopore; it is here that the differentiation of ectoderm from endoderm begins. ‘therefore we $ e.g. conklin, ‘' the effects of centrifugal force on the organisa- tion and devclopment of the eggs of tresh-water pulmonates,” jour, exp. zool., vol. 9 (1910). °f, w. r. brambell, ‘‘ the nature and origin of yolk,” british jour. exp, biol., vol. 1 (1924). 1 brachet, joc. cit. aw, roux, ‘' uber das entwicklungsmechanische vermegen jeder der beiden ersten furchungszellen des eies,” 6th. verhandl. der anat, gesellschaft (1892). 2. hertwig, ‘‘ uber den werth der ersten furchungszellen ftir organbildung des embryos,” arch. f. mikr. anatomie, vol. 42 (1893). 4 brachet, loc. cit. * 974 conclude that the arrangement of the organ-forming substances in the frog’s egg is caused by the spermatozoon. in the mollusc dentalium when the egg has reached the four-cell stage one of the blastomeres emits a protuberance termed the “ yolk- lobe ” or ‘‘ polar lobe.” this lobe is devoid of a nucleus and before the attainment of the cight-cell stage is reabsorbed into the blas- tomere. nevertheless, if this lobe be cut off, the remainder of the egg develops into a larva which is fatally devoid of mesoderm. that the materials which form the basis of the different substances embodied in the germinal layers are formed in the growing egg under the influence of emissions from the nucleus is rendered certain, first by the close relationship of the nucleus to assimilation and secondly by the fact (see cyrolocy) that the nucleolus of the unripe egg breaks up into fragments and is extruded into the cytoplasm. it is, however, a surprising fact that the nuclei of the segmenting eg¢ are alike and apparently without influence on the differentiation of the primary organs. in fertilisation a second nucleus of alien origin is introduced and portions of this nucleus, as we have already seen, are incorporated in all these ‘‘ segmentation ”’ nuclei. now it is common knowledge that the tntluence of the father is as potent as that of the mother in heredity and therefore there must arrive a period of development at which the nuclei again influence the cytoplasm. an attempt to determine this period was made by macbridet by fertilising the eggs of echinocardium with the sperm of echinus, the result of this cross is in most cases to produce cytolysis of the egg, but in a minority of cases a hybrid develops. the egg of echinocardium is oval whereas that of echinus is spherical and the shape of the blastula of each species follows that of the egg. the blastula of the hybrid is oval, like the maternal blastula, and the gastrula is also like that of echrnocardium. but the typical larva (the four-armed echinoplutcus) resembles in several points the larva of chinus; in the vast majority of cases it is totally devoid of a large aboral club supported by a special skeleton which is characteristic of the larva of echinecardium. it is clear therefore that at this stage the paternal nucleus is influencing the structure of the organism. when the eggs of echinus are fertilised with sperm of a still more divergent character, such as that of the crinoid anjedon, a hybrid occasionally develops as far as the gastrula stage, but it always resembles the larva developed from the normally fertilised egg in every detail and shows no trace of paternal influence. nuclei and cyteplasm.—wwe are thus led to the conception of an intermittent action of the nuclei on the cytoplasm, and in this it seems as if we had reached the deepest point to which analysis of development has so far led us. perhaps it would be more accurate to speak of an intermittent reaction between cytoplasm and nucleus, for in some embryos there is evidence that the nuclei undergo altera- tion as development proceeds. it is on cases like these that weis- mann’s theory of development was founded.? according to this theory, as growth proceeds, differential division of the nuclei takes place, some becoming specialised as ectodermal nuclei, others as endodermal nuclei, and so on, whilst some retain the constitution of the original zygote nucleus. these last give rise by division to others like themselves which eventually engender the nuclei of the germ cells. the lineage or line of descent leading from these germ-cell nuclei back to their ancestors amongst the nuclei of the first blas- tomeres is termed the “‘ germ-track.” now in the nematode worm ascaris megalocephala the zygote nucleus contains only four chromo- somes, but as the egg divides into blastomeres the nucleus of one blastomere after another undergoes the change termed diminution of the chromatin. this change involves the nipping-off of the ends of the chromosomes, and these portions are ejected into the cytoplasm and are absorbed; the remainder of each chromosome becomes frag- mented into a large number of minute granules. these granules act as chromosomes in the next nuclear division. the nucleus of one blastomcre remains ‘exempt from this change and this blastomere eventually gives rise to the genital organs. boveri? has shown that the fact that one nucleus undergoes diminution of the chromatin whilst another does not is not the con- sequence of a differentia! division of the mother nucleus of them both, but is due to the fact that one nucleus takes up its position in a region occupied by a particular cytoplasmic substance. this he po in two ways, viz.: (1) by considering the case of eggs fertilised y two spermatozoa, and (2) by the results obtained by subjecting eggs about to segment to the action of strong centrifugal force. in doubly fertilised eggs the extra spermatozoen forms an inde- pendent nucleus whilst the other fuses with the female pronucleus to form the zygote nucleus. the first division of the egg results in the formation of four nuclei and four blastomeres. in the development of the normally fertilised egg one of the two first nuclei undergoes diminution, and the cell containing it gives rise to a large part of 1e. w. macbride, ‘ studies on‘the development of echinoidea: (ii.) the early larva of echinocardium cordatum and the result of crossing this species with echinus escudentus,"’ quar. jour. micr. science, vol. 58 (1912). 2a. weismann, the germ-plasm. a theory of heredity (1900). § th. boveri, ‘‘ die potenzen der ascaris- blastomeren bei abge- fnderter furchung," festschrift sum oosten geburtstag richard hert- wigs, vol. 3, no. 8 (1910). embryology the dorsal ectoderm: the other nucleus remains undiminished and amongst the progeny of the cell containing it are found the genital cells. now amongst the four cells produced by the division of the doubly fertilised egg, three may contain nuclei which undergo diminu- tion, and one may remain undiminished and in such cases the egg develops into a single embryo with an unusually abundant ectoderm. in other cases only two of the nuclei undergo diminution; such eggs form twin embryos of normal aspect; whereas in still other cases one nucleus alone may undergo diminution and in these cases a mon- strous triple embryo is formed. these differences are accounted for on the assumption that one region of the egg contains a substance which induces diminution and one, two or three nuclei of the doubly fertilised egg may be in it. when eggs about to segment are exposed to the action of long- continued and intense centrifugal force the plane separating the first two blastomeres will in some cases be found to lie along a radius of the circle of rotation, and in these cases a small mass of material will be found to be ejected from the egg which then becomes divided into two appreciably equal and similar blastomeres, the nucleus of neither of which undergoes reduction. this suppression of reduction must be attributed to the even distribution of the cytoplasmic ma- terials under the stress of the centrifugal force, so that no region of the egg contains more of the peculiar substance than any other. diminution of the chromatin apparently results from the action of an excess of this substance on any nucleus contained in it. regencration.—in the phenomena of regeneration and of bud- ding we meet with evidence of the renewed influence of the nuclei in causing the formation of cytoplasmic substances. when one of the first two blastomeres into which a frog’s egg divides is killed the survivor frequently develops into a half gastrula which may even grow into a half tadpole. roux’, however, has shown that if this half tadpole survives it becomes a whole tadpole by what he calls the “ post-gencration " of the missing half. this is effected by the multiplication of the cells lying at the edges of the half embryo. the nuclei increase in number and confer on the cyto- plasm in their neighbourhood new powers. in this case it might be objected that each kind of tissue in the old half gives origin only to the same kind of tissue in the new half. but morgan’ has shown that if the head (including the pharynx) of the annelid nereis be cut off, a new head with pharynx will be regenerated from the stump; whereas, however, the original pharynx was formed by an intucking of ectoderm, the new pharynx is formed by an outgrowth from the endodermal tube in the stump. the new powers thus conferred on the cytoplasm of the endodermal gut can only be explained as the result of the calling-forth of new potentialities in the nuclei lying in the cut edge. more remarkable evidence still has cropped up in connection with the regeneration of the lens of the eye of the newt. in the embryo the lens is formed as a thickening of the ectoderm on the side of the head. but if the original lens be torn out, a new lens is developed from the edge of the retinal portion of the iris—a tissue which has no connection with the skin of the head. some try to meet this difficulty by the phrase that in these cases the organism acts as a whole, independently of the germ layers into which we analyse it. but what meaning can be attached to this phrase, except that the organism under different circumstances uses different means in order to effect a restoration of its integrity, it would be difficult to say. in fact we approach very closely to the celebrated ‘‘entelechy ” of driesch*; that is an indwelling ‘‘ something ” in an organism which strives to realise a purpose. vitalism and the theory of an entelechy.—it may be argued that such an idea is unscientific, because it introduces ‘‘ vital force ’’ and similar mystic ideas amongst our biological conceptions. [t¢ may be answered that in the last resort a// explanation is comparison, and that those who reject vitalism seek to compare all the activities of living beings to phenomena which go on outside the body in test-tubes. but this is equivalent to referring all the phenomena of life to struc- ture, in other words to the juxtaposition of definite chemical sub- stances in a definite spatial arrangement; in regeneration, however, we encounter phenomena where structure appears to be irrelevant. if we are to do justice to such phenomena we must have some work- ing hypothesis similar to that of driesch. whether the assumption of an ‘'entelechy " is better or worse than the statement that all the nuclei in the body are totipotent and that varying potentialities are called forth in different cases seems to be a matter of taste. budding. —regeneration is in many respects akin to budding, since buds in many cascs may be regarded as portions of the mother organism restored after natural amputations. in the growth of buds we often meet with a wide divergence be- tween the matcrials used to build up certain organs and those used to construct similar organs in the embryo. to give an example—the bud of the ascidian boiryilus begins its existence as a little two- layered vesicle very similar to the gastrula of the same species. but 4 roux, foc, cit. ’t. h. morgan, regeneration (1901). 6h. driesch, zwei vortrige zur naturphilosophie (1910); see also gifford lectures for 1907 and 1908. embryology 9795 in the embryo the central nervous system is developed from the | that the right hydrocoele bud can totally alter the development of outer layer as it is in all other vertebrata, in the bud, on the con- trary, it is formed as an outgrowth from the inner layer. hjert,! who described this phenomenon, suggested as the explanation for it the fact that the outer layer of the bud is an outgrowth of the adult maternal ectoderm, which is specialised for the secretion of the cellulose ‘“ mantle” and not sufficiently plastic to be turned into nervous tissue. this is only another way of saying that the formative nuclei act differently in different cases and distribute the organ- forming cytoplasmic substances in a different manner in the bud from their arrangement in the egg, one or two embryos.—the primary organs, 7.e., the germ layers, are the material out of which the higher organs are built up, and one of the most remarkable of recent discoveries in em- bryology is the fact that the question of whether this material shall be used to build up one embryo or two depends on the special relations which these primary cytoplasmic substances sustain to one another. if the eggs of a frog be placed dry on the surface of a slide with their animal poles uppermost and fertilised in that position by the addition of small quantities of the fluid extracted from the seminal vesicles of a male; 1f then another slide be placed on top of them and the two slides clamped together by rubber bands; if when the eggs have divided into two blastomeres the whole preparation be inverted and left in water in a shallow dish for five or six days tadpoles with two heads or two tails will be developed. the materials in the un- segmented egg are of different specific gravities; the first furrow often (see above) divides them into two symmetrical halves; when the two- cell stage is inverted they tend to re-arrange themselves in each cell in the same manner as they would have done in the whole egg had it been inverted. nothing has been added or taken away, yet the al- tered position of the materials in each cell has led to the formation of two organs where normally only one would have been formed. in the case of the newt’'s egg a similar procedure leads to the formation of two complete embryos, whilst if the blastula of the newt be con- stricted longitudinally by a hair a two-headed monster is formed. when a lizard’s tail is broken off, if the little regenerating bud which forms at the wounded surface be indented the animal will regenerate two tails instead of one, internal environment.—when the higher organs begin to de- velop we can in many cases prove that the whole course of their growth is governed by what may be called their internal environ- ment, z.e., by influence emitted by other organs. this may be clearly seen in the development of the common sea- urchin echinus miliaris, the ‘‘ echinoplutecus "’ larva of this species is a transparent bilaterally symmetrical free-swimming creature. it is provided with a complete alimentary canal consisting of oesoph- agus, stomach and rectum, and at the sides of the oesophagus are situated two flattened coelomic sacs. as development proceeds each sac becomes divided into anterior and posterior portions, and the latter move backwards so as to be pressed against the stomach. still later from the posterior end of the left anterior sac a little bud termed the “ hydrocoele ’’ grows out. this is the rudiment of the water- vascular system of tubes in the adult. the ectoderm lying over this bud becomes depressed so as to form a sac (the “ amniotic cavity '’) from the floor of which grow up the spines which will cover the test of the future sea-urchin. the hydrocoele bud overlaps the front end of the feft posterior sac, and from this part of the posterior sac there grow out five pockets from which will be developed the dental apparatus—the so-called “* aristotle's lantern.” from the outer wall of the right posterior coelomic sac cells are given off from which are developed a pair of “ pedicellariae "’ (pincer-organs) which will be situated on the upper surface of the future urchin. if we now allow? the young larvae at the time the coelom is being formed to grow in hypertonic water, then many of them will develop from the right anterior coclom a sec- ond hydrocoele bud. if this bud develops—and it does so if plentiful nourishment be supplied to the larva—then a right amniotic cavity is formed from the overlying ectoderm, whilst the right posterior coelom gives rise to a second aristotle’s lantern. if the development of the second hydrocoele bud be slow, then one or even two pedicel- lariae may be formed on the right side as in normal larvae, but if it be rapid the formation of pedicellariae may be inhibited altogether. if after the bud has appeared the larva is nearly starved for a time, both this abnormal bud and the normal hydrocoele may remain small and undeveloped and then pedicellariae may be formed on the left side as well as on the right. we conclude from these facts that the hydrocoele bud tends to inhibit the formation of pedicellariae on its own side of the larva but to cause their production on the opposite side, and we see further 1], hyjert, “ germ-layer studies based on the development of ascidians,” zool. results norwegian n. atlantic exped. (1890). 2f, w. macbride, “‘ the artificial production of echinoderm larvae with two water vascular systems,” proc. roy. sec. (london), sexies b, vol. 90 (1918). the right side of the larva, forcing the right ectoderm to form an amniotic cavity and the right posterior coeclom a dental apparatus. another still more striking case of the influence of the internal environment is afforded by the results of experiments performed on the tadpole of the frog. the vertebrate eye consists of two main parts, viz.: (a) the retina, formed as an outgrowth from the brain; and (4) the lens, formed as a thickening of the ectoderm of the side of the head. uf before the lens is formed the skin of the head of a tad- pole be slit open and the retina cut off from the brain and pushed back till it occupies a position in the region of the shoulder or even farther back, the slit in the skin will soon heal up and then the tad- pole will recover; the cut-off retina will continue to live and grow in its new position, and it will force the ectoderm covering it to form a lens—although never in the history of the race has a lens been nor- mally formed in this position. numerous other similar instances could be adduced suggesting the conclusion that in many embryos the primary organs are indifferent material and that the manner in which the secondary organs will develop out of them is fundamentally a matter of their spatial relations. we shall, however, confine ourselves to citing one more case of this phenomenon, the most striking that has yet been discovered. we have already mentioned the fact that the differentiation of organs in the frog’s egg begins in the upper lip of the blastopore. this is also true of the eggs of newts and apparently of all other amphibia. spemann* removed a small piece of this upper lip from the gastrula of the newt triton taeniatus which is deeply pigmented ancl implanted it in the midst of the lateral wall of the gastrula of the newt fyiten cristatus which ts devoid of pigment. the small dark piece of foreign tissue effected a complete union with its alien sur- roundings and as development proceeded a nerve cord was developed in front of it from the tissues of the host, entirely independent of and supplementary to the proper nerve cord of the host which developed in the usual way from the dorsal legs of the blastopore. here we have the clearest demonstration of the fact that all the potentialities of the development of the entire embryo slumber in each cell and can be awakened if the appropriate stimulus reaches them, a stimulus which in this case is produced by the tissue of another species. external environment.—we now approach the subject of the possible influence of the external environment on the course of development. in the earlier article attention was drawn to the fact that development presents itself under two principal aspects, viz., the embryonic and the larval. in the embryonic phase the young organism is sheltcred from the external world, either within an egg-shell or in the mother’s womb, whereas in the larval phase it leads a free life, using its larval organs to seek its own food and escape its enemics. it was further pointed out that if we compare two nearly allied animals such as sulamandra atra and salamandra maculosa, in the first of which development is mainly embryonic, whereas in the sccond it is largely larval, we arrive at the conclusion that the embryonic phase is secondarily derived from the larval phase, since the organs such as gills which are functionless in the embryo are functional in the larva. it was also pointed out that larval organs frequently resemble the adult organs of other animals of simpler and more primitive structure. on these facts was founded the celebrated biogenetic law first enunciated by haeckel which affirms that “‘ the embryo in its development recapitulates the ancestral history of the race.” it is this law which provides a large part of the fascination of embryological research, though it was vigorously attacked in the earlier article when an effort was then made to show that it is not valid, since it was maintained that whilst it is true that larvae retain ancestral characters, the same is true of adults, and that larvae in their structure are not more reminiscent of the former history of the race than are adults. now the outcome of recent investigation has in large measure tended to reinstate the doctrine of recapitulation in its former position of pre-eminence, to show in fact that recapitulation forms the central thread in every life history, although it has been blurred and deflected by secondary influences, as indeed all believers in the biogenetic law have from the first admitted. the first point to which we wish to direct the reader's attention is that larval and embrvonic phases occur in all life histories. every 3w. h. lewis, ‘ studies on the development of the eye in amphibia. 1. the lens,” american jour. anat., vol. 4 (1904). 4h1. spemann, “die erzeugung tierischer chimaeren durch heteroplastische embryonale transplantation zwischen 7riton cris- tatus und taeniatus,” arch, f. entwickliungsmech. vol. 48 (1921). 6. haeckel, generelle morphologie (1866). 976 animal begins its existence as an egg which is quite incapable of feeding or of defending itself and this egg is always protected by an egg-shell although this shell may be very thin, and no animal upon leaving its early shelter and beginning to seek its own food attains at once the structure of the sexually ripe adult. hence every animal in the course of its development may he said to pass first through an embryonic and then through a larval phase, although the latter phase may be very short and the difference in structure between the larva and the adult inconsiderable. now, the larval phase being the later is the most recent addition to the life history, and therefore the least likely to be modified by secondary factors; if therefore the biogenetic law is valid, it is the larval phase which will possess most ancestral significance. but in the earlier article attention is called to the fact that the identification of a larva as the representative of an ancestor must always be hypothctical because we have no direct knowledge of what the ancestor of any living animal was like. it bchoves us there- fore to look a little more closely at the reasons which actually do induce us to regard a given stage as ancestral. first, we may call attention to the fact that quite recently direct experimental proof of the validity of the biogenetic law has been obtained. kammerer! placed young specimens of sa/amandra macilosa which had just completed their metamorphosis in cages the floors and walls of which were coloured diiferently in different cases. the larva of this species has a skin of a uniform dark-greyish tint but the skin of the adult is gaily coloured with bright yellow patches on a black background. the salamanders which were confined in cages having a floor of moist yellow loam and walls coloured yellow became yellower as they grew to maturity—a process which occupies between four and five years. the yellow patches increased in num- ber and size and tended to become joined together in bands. those confined in cages with blackened walls and a floor of black garden earth became darker since the yellow patches dwindled in size. when the salamanders had attained sexual maturity and were allowed to pair, it was found that the offspring of two which had been reared in yellow surroundings, if they continued to live in the same environment, became still yellower than their parents until the black pigment had been almost entirely displaced; whilst the offspring of two which had become darker, if reared in cages with black walls and floor, became practically completely black by the time they reached maturity so that they came to resemble the mountain species salamandra atra. if, however, the offspring of two salamanders reared in yellow surroundings were allowed to grow up under black surroundings, they nevertheless for the first six months of their lives became progressively yellower; then and only then did the influence of the black environment begin to tell—the yellow patches became invaded by numerous small black spots and grew smaller. in short, the young recapitulated the process of “ yellowing " that their parents had undergone. if these results are confirmed the doctrine of recapitulation will change its status from that of an hypothesis to that of a proved fact; and further proof will be furnished that changes acquired by the individual in response to the demands of the environment are to a certain extent at jeast inherited. the recapitulation theory.—once we have grasped the mutual relationship of the embryonic and larval phases of development, indirect proofs of the reality of recapitulation begin to crowd in on us. if we find, for instance, one or two aberrant forms in an order or even a family the majority of whose members have a uniform type of structure, no reasonable doubt can exist that the ancestors of these aberrant forms had the typical structure of the group. if this conclusion be admitted and we find that the younger stages of the aberrant spcecics also show the typical structure, does any one seriously question that these young forms recapitulate the history of the race? two very striking instances of this kind have come to ight within the group ctenophora. the typical! ctenophora are ovoid organisms of a glassy trans- parence which swim in a vertical position in the sea. their loco- motor organs are eight vertical rows of vibratile combs, each comb consisting of a short horizontal row of powerful cilia fused together at their bases. a certain creeping organism resembling a flat worm, named coeloplana, had been believed by some zodlogists to exhibit ctenophore affinities but its relationships were very obscure. quite recently a japanese zoglogist? has described its development. its larva is a small typical ctenophore with eight rows of perfectly formed combs; these it discards after swimming for a few hours—it sinks to the bottom and hattens out and gradually assumes the adult struc- ture. another extraordinary organism, named by its discoverer tjalfjella,3 was discovered amongst dredgings collected in the arc- 1p. kammerer, ‘‘ vererbung erzwungener farbveranderungen, {v. das farbkleid des feuersalamanders (salamandra maculosa) in seiner abhangigkeit von der umwelt,” arch. f. entwicklungsmech., vol. 36 (1913). . 2 taku komai, ‘ notes on coeloplana bocki and its development,” annotationes zoologicales japonenses, vol. 9 (1920). 3 \[ortensen, ‘“‘ ctenophora,”’ danish ingolf expedition, vol. 5, no. 2 (1912). embryology tic ocean. this creature superficially resembled a sponge or an ascidian. it was gelatinous and sessile and scemed to consist of a pair of upright tubes like towers whence proceeded smaller tubes which ramified in its substance. in pockets connected with these smaller tubes were discovered groups of the larvae. these were small ovoid creatures of typical ctenophore structure with the cight vertical rows of combs. change of itabits.—if recapitulation of ancestral history forms an unquestionable clement in the life history of some animals, it is probable that it constitutes a factor in al} life histories. what is, then, its meaning? the recapitulatory element is most obvious in the latest larval stage of development, the most recently added page of the life history. now the organs of the larva are adapted to its environment; therefore this environ- ment in its broad outlines at least must represent the ancestral environment of the race. the present condition of the race both as regards structure and habits has been produced as a consequence of migration from the original haunts of the race. change of habits therefore reveals itself as the great driving-force in evolution, and change in habits usually means the choice of a different type of food. we may conclude that the period of life at which this change most frequently occurred was when the adult organs had de- veloped but before sexual maturity had been attained—in a word, at the stage of what we may call the young adult. as one change of habits succeeds to another in the course of evolution, the life history is not lengthened in the same proportion, since the new phase takes the place of the sexual phase in the previous condition of the race. in some crustacea, e.g., in the shrimp penaeus, at least four larval stages are passed through before the adult stage is attained, but in the majority of life histories when a new phase is added there is a tendency for some of the older phases to be pushed back into the embryonic period, so that as an animal passes from stage to stage in evolution it leaves behind a trail of stages at first larval and then becoming embryonic. secondary modifying factors —the principal factors which modify and tend to obscure the recapitulatory factor can only be defined by a truly comparative embryology based on a wide survey. | one of these factors is ‘ tachygenesis ’’ or precocious develop- ment; that is to say, we find that organs originally developed as a response to the stimulus of a new environment come in course of time to be developed before the habits to which they correspond can be exerciscd—in fact acquired habits tend to become innate. thus the young hermit crab when adult thrusts its abdomen into the cavity of a spirally coiled gastropod shell, and in this way imposes a twisted form on this part of its body. but if all such shells be removed from the hermit crab’s neighbourhood at the time of its metamorphosis, it will still develop a curved abdomen, although the extent of the cur- vature will be less than that which occurs normally. when the tad- pole of the frog acquires limbs, these do not develop in the form of fins, from which they have been undoubtedly evolved, but grow directly into the ordinary type of five-toed limbs, although weeks must elapse after their form is fully defined before they can function as the limbs of land animals. the tendency to hurry on develop- ment may be compared to the increasing facility with which a difficult operation 1s performed after long practice, but this tendency obviously obscures the distinctive features of early development. a second powerful modifying factor is the change from the larval to the embryonic phase, so far as the development of a particular organ is concerned. this change of phase is sometimes caused by an unfavourable alteration in the environment of the larva. it was actually effected artificially in the development of salamandra macu- fosa by kammerer.4 this species is viviparous and normally gives birth to between 30 and 40 young which are provided with gill- slits and long gills and which live in the water for six weeks before they metamorphose into land animals. if the parents are exposed to successively colder and drier conditions, the number of young produced at a birth diminishes with each breeding period, and these young are born at a progressively more advanced stage of develop- ment. if these young are reared to maturity under similar conditions of coolness and dryness, they will in turn give birth to young which will be still fewer in number than those produced by their parents and which are born at a still more advanced stage of development. the process gocs on till only three or four are born at one time and these are provided with the merest stumps of gills; such young never #p, kammerer, ‘‘ vererbung erzwungener fortpflanzungs- anpassungen. i. & ei]. die nachkommen der spatgeborenen sala- mandra macilosa und der friihgeborenen s. efra,” arch. f. entwick- lungsmech., vol. 25 (1908). oe embryology enter the water at all but at once take up the adult mode of life. this is the normal mode of development of salamandra atra. the change of phase from the larval to the embryonic type entails many other changes. the embryo must be fed and it obtains its food from one of three sources, (a) devouring its sisters; (5) secre- tions from the mother’s womb; (c) inclusions of yolk in its own ecto- plasm. when the embryo devours its own sisters, this, as in the case of salamandra atra, may entail little change of structure because in this case the habit has been recently acquired; but where, as in the case of the platyhelminth worms, the habit is of old standing, then the embryo may be distorted out of all recognition. in these worms one viable egg is shut up in a capsule along with thousands of small stcrile ones; and it is difficult to find in the embryo any vestige of resemblance to the larvae of those platyhelminthes which lay their eggs singly. when the embryo derives its nourishment from the mother's womb, then it frequently develops organs of achesion to the wall of this. to this category belongs the placenta which profoundly dis- torts the ventral surface of the human embryo, so that this surface gives rise to a treelike outgrowth whilst the dorsal surface is moulded into a ludicrously exact copy of the early tadpole of the amphibian. when the embryo is fed by yolk, this, as we have already pointed out, modifies all the processes of development; cell division becomes slow and the cells produced few and large, and folding, which plays a large part in the development of small alecithal eggs, becomes im- possible and is replaced by solid outgrowths of cells. still a third factor which tends to hide the recapitulatory element is the development of special larval adaptations. this occurs when the larva retains its free life but when its circumstances become changed, these special adaptations have been developed in thou- sands of insect larvae. so gencrally is this the case that balfour! denied to these larvae any ancestral significance at all; but modern research has succeeded in revealing the original ancestral larval type beneath the secondary modifications. all the evidence at our disposal points to the conclusion that the ancestors of insects were creeping myriapod forms—scavengers which fed on the debris of both animal and vegetable nature in the undergrowth of primeval forests. such is in fact the life of the lowest insects known to-day, some of which, such as machilis, nourish them- selves on the decaying seaweed on the seashore, and retain through- out life vestigial limbs attached to the abdominal segments which aid them in their crawling movements. now the myriapod or poly- pod larva survives as the caterpillar of the lepidoptera mee the primitive hymenoptera. it is also found amongst the primitive neuroptera and amongst the may-flies (ephemeroptera). these last-named insects were supposed to possess a larva showing great secondary modifications, for it is provided with icaf-like gills attached to its abdominal segments; but heymons? has shown that these gills are nothing but modified abdominal legs. it is a curious fact, how- ever, that amongst the lower insects, such as the cockroach, this polypod stage is usually passed through during the embryonic phase of development. the reason for this change seems to be that these insects lay their eggs in situations where a grub-like larva would perish, whereas the higher insects, in which the stage is larval, are gifted with instincts which lead them to lay their eggs in situations where an abundance of easily procured and easily masticated food is available and a scavenging existence like that of the ancestor is possible, a fourth factor which modifies development, and which is potent in its effect although it is usually overlooked, is loss of size in the lar- va as compared with the ancestor which it represents, as conditions change and the larval life becomes more dangerous there arises a tendency in which we may trace the influence of tachygenesis to pass quickly through the larval stage and to metamorphose at as early a period of growth as possible into the adult condition. thus in the american lobster the adult stage is attained when the animal is 14 in. long but this lobster eventually attains a length of from 18 in. to two feet. a consequence of this change is that the larva assumes a new relation to its environment, for many qualities of the surrounding medium, such as the viscosity and the supporting- power of water, acquire an altered importance as the organism de- creases in size. if the ancestral organs were reduced in the same scale as is the whole body of the larva, this would often result in their becoming incapable of being used. as a consequence we find that in many cases where the ancestor had a series of organs, this series ig represented in the larva by fewer members or only one member of larger relative size, and that where in the ancestor there was a pair of organs there is frequently only one in the larva, but this is on a larger scale than the rest of the body. if we now sclect a few examples to illustrate this principle, we may consider the free-swimming larva of that most primitive of all vertebrates amphioxus. this larva has only a single scrics of gill- slits which are so enlarged as to occupy the whole ventral surface of the body. if the double series of slits, which the ancestor in common with all other fish possessed, had been developed in the larva, they would necessarily have been of such minute size that the capillarity 1f, m. balfour, comparative embryology, vol. 2, p. 365 (1881). 2r. heymons, “ uber die lebensweise und entwicklung von ephemera vulgata,” sitzungsber. d. gesells. d. naturf. berlin (18096). — 977 of the water would have prevented them from being functional. similarly there is no doubt that the eyes of vertebrates were from the beginning paired structures, but they are represented in the ascidian tadpole by a cup-like outgrowth of one side of the brain. again no serious doubt can be entertained that the primitive arthropod was evolved froma long many-segmented annclid with flexible parapodia. but the most primitive larval form of the crustacea is the nauplius which in most cases is a little, oval, unsegmented creature with but three pairs of legs. in some of the most primitive crustacea, how- ever, such as estheria and chirocephalus, the nauplius possesses when hatched a comparatively long ‘abdomen which though devoid of appendages is divided into a series of narrow ring-like segments. the ordinary type of nauplius larva seemed to indicate that the crustacea must have been derived from an unsegmented animal totally distinct from the ancestor of other arthropoda, for the progenitor of these must have been long and segmented, since the embryos of all these arthropoda have many segments. but if we look at the nauplius larva from the standpoint of func- tion rather than of structure we have no difficulty in seeing in it the recapitulation of the first step in the ‘“ arthropodisation”’ of the annelid. this step was a change of habits which consisted in using the foremost parapodia as oars to propel the animal and as organs to seize food. asa consequence in the front of the body the cuticle was thickened and the “ arthropodous ” type of limb produced, whilst in the rest of the body the annelid condition of affairs persisted as indeed it may be almost said to do’ in the posterior portions of the bodies of this most primitive crustacea, the long-bodied phyllopoda such as artemia. once begun in front, this ‘‘ arthropodous ”’ modi- fication was gradually propagated backwards so as to involve the hinder segments of the body and in this way the higher crustacea were evolved. in the nauplius larva, the anterior arthropodised portion of the ancestor with its appendages is alone represented; the hinder annelid portion in which function was less intense and less important is feebly or not at all developed. one last instance of the principle may be adduced which we sclect from the embryology of the higher vertebrates. in the devel- opment of those types of vertebrata in the life history of which there is a long larval phase (cyclostomata, dipnoan, “ ganoid " and teleos- tean fish, amphibia), a larval excretory organ termed the pronephros makes its appearance. its duct later becomes the duct of the permanent kidney, but the pronephros itself consists of very few tubules and these originate from the wall of the general body-cavity and not, as do the tubules of the permanent kidney, from special sacs (the malpighian capsules). the earlier workers regarded the pronephros as a last trace of a primeval excretory organ of quite different structure from the permanent kidney which later super- sedes it. [latta* working on the devclopment of the lamprey and kerr? on the development of polypterus, have proved that the pronephros is nothing more than the foremost section of the perma- nent kidney, early called into action and enlarged whilst the hinder section of the metamerically repeated series of tubes of which the kidney consists remains undeveloped. further, the portion of the general body-cavity from which the pronephric tubules arise is proved to consist of several malpighian capsules fused together and secondarily communicating with the gencral body-cavity. life itistory of animals.—it might be supposed that with so many modifying factors at work it would be a hopeless task to attempt to disentangle the recapitulatory element from them and that therefore the ancestral history of animals, except in its latest and icast modified chapters, would remain a closed book. but when we recollect that the hfe history of every species con- stitutes a separate edition of this history, and that the modifying factors have affected no two of them to the same extent, it be- comes evident that comparative embryology built on a broad basis can attack the problem with a fair prospect of success. bearing in mind the priority of the larval over the embryonic phase, and beginning therefore our survey with the larvae of the simplest metazoa, we areable to recognise the first step in the evolution of the metazoa from the protozoa in the blastula, the hollow ball of cells which may be regarded as representing a colonial protozoen like the living volvox. this stage was followed by the formation of a gut by the intucking of one side of the ball; and this second step is represented in the life histories of all the jower and simpler animals by the gastrula stage. following on this stage came the formation of the coclom as a series of pouch- like outgrowths of the gut, and the change of the single opening of the gastrula, the blastopore, into two openings which became the mouth and the anus by the constriction of its middle portion. it has been possible to show that two groups so utterly diverse 4 hatta, the development of the renal organs in the lam- prey,” journ. coll. sci. imp. univ. tokio (1912). 4j. graham kerr,’ vertebrata with the exception of mfammalia,” textbook of embryology, vol. 2 (1919). a 978 in appearance as the annelida and the mollusca have originated from a single group of free-swimming ancestors represented by the trochophore larva, and since arthropoda are admitted by all to be descended from annelida, this conclusion involves the ancestry of four-fifths of the animal kingdom. we can form a very plausible guess as to the nature of the diver- gence of habits which led to the differentiation of the annelida and mollusca from one another. the original stock was free-swimming but both groups derived from it are typically bottom-dwellers. two modes of seeking their food were open to them: they could cither glide over the bottom by means of their cilia as young gastropoda and lamellibranchiata still do, or they could burrow into it. the first of these habits led to the evolution of mollusca, the second to that of annelida. two other groups of very diverse structure, which embryology has given strong reasons for believing to have been derived from a single race, are the echinodermata and the vertebrata. the lowest form which gives distinct evidence of the vertebrate affinities is the worm- like creature balanoglossus. the larval form of balanoglossus is a free-swimming organism called ternaria which shows the closest resemblance to the typical larva of echinodermata, the dipleurzula. the recognition of this affinity has assisted in the elucidation of a difficult subject to which considerable space was devoted in the 11th ed., viz., the origin of the central nervous system. it is characteristic of the most primitive annelida and arthropoda that this system develops as a ring round the blastopore and an en- deavour was made in the rth ed. to prove that this was originally true of vertebrate embryos also. but it is a peculiarity of the verte- brate-echinoderm alliance which is still unexplained that in them the blastopore gives rise to the anus alone, whilst the mouth is formed as an apparently independent perforation at a considerably later period of development. a long succession of embryologists, with their eyes fixed only on the embryos of annelida, arthropoda and the higbee vertebrates, have held that the vertebrate mouth is a new structure formed by the fusion of a pair of gill-slits and have prosecuted vain searches for traces of the old mouth. others have imagined that the mid-dorsal line of the vertebrate embryo along which the nerve-cord develops corresponds to the line joining mouth and anus in the arthropod, the line in fact which is occupied by the slit-like blastopore in peripatus, they hold that the nervous system of the vertebrate originally extended round the front end of the embryo so as to include the mouth. there is, how- ever, strong evidence that in vertebrata the blastopore originally extended along the ventral surface. in amphioxus the blastopore 1s at first a large wide opening beneath the nerve, the diameter of which is at right angles to the long axis of the nerve cord. as development proceeds this opening closes from below upwards and the blastopore is finally reduced to a small pore immediately posterior to the nerve cord. the anus arises as an apparently new perforation along the line of this closure. in the newt the lower part of the blastopore remains permanently open as the anus. in elasmobranch fish (sharks and rays) the blastopore is at first an enormous opening occupying the whole under surface of the egg which gradually closes by the approximation of its sides. as the anus can develop as an apparently independent opening along the line of the closed blasto- pore, it does not require a great stretch of the imagination to assume that this line really extends so far forward as to include the area where the mouth later develops. the independent and late develop- ment of the mouth is as marked a feature in the echinoderm larva as in the vertebrate embryo and there are no gill pouches in the echinoderm on which we could fall back to explain the phenomenon; and, further, in all four types of echinoderm larvae develop a wide mouth leading into a shallow funnel-shaped gullet situated in the same position in the larval body and showing no relation whatever to the blastopore. in two groupsof echinodermata (ophiuroidea and holothuroidea) as development proceeds this mouth moves to the left: its inner por- tion forms the adult mouth whilst its outer portion shallows out and forms the circular lip (peristome) surrounding the adult mouth, in two other groups (asteroidea and echinoidea) the larval mouth entirely disappears and the adult mouth is formed as an independent perforation on the left side—yet no one would qtiestion the homology of the mouth in all four groups. these facts afford conclusive proof that the shift of an organ hon one position to another may be represented in embryology by its disappearance in one position and its apparent new formation in another, in the adult echinoderm the whole of the ectoderm is underlain by a nervous plexus of which the central nervous system is only a specialised and intensified portion, and the same thing is true of the anterior region of balanoglossus. it is therefore futile to look for exact correspondence between the central nervous systems of two stocks which diverged from one another at such a primitive level as did the vertebrata and the annelida. in fact the descendants of the trochophore stock (anne- lida, arthropoda and mollusca) on the one hand and the original vertebrata on the other seem to have adopted two different modes of life which led to concentrations of the nervous system in different parts of the body. the trochophore stock took to crawling on their ventral surfaces and their locomotor organs were developed in this emigration—encephalitis lethargica region of the body and in connection with them the motor ganglia which make up the ventral nerve-cord; whereas the vertebrate stock took to swimming by lateral blows of their blade-like bodies and this lel to the concentration of the central nervous system in the mid- dorsal line. . enough has been said to give evidence for our belief that the most recent research has tended to re-establish the recapitulatory element as the fundamental factor in life-history, and if this be admitted the study of comparative embryology opens up a means of investigating the early history of life at a stage long before it left evidence of its existence in the stratified rocks; and, further, the acceptance of recapitulation involves a conception of the laws of heredity entirely distinct from and supplementary to that sug- gested by gregor mendel and his followers. bibliography in addition to works quoted in notes.—a. milnes marshall, vertebrate embryology (1893); e. korschelt and c. heider, textbook of the embryology of the invertebrates, translated by e. mark, ph.d., and w. mcm. woodworth (1895); o. hertwig, lehrbuch der entwicklungsgeschichte des menschen und der wurbelthiere (jena, 1906); j. c. heisler, textbook of embryology (1907); h. driesch, the science and philosophy of the organism; gilford lectures for 1907 and 1908; f. r. bailey and a. m. miller, textbook of embryology (1909); j. w. jenkinson, experimental embryology (1909); vertebrate em- bryology (1913);three lectures on experimental embryology (1917); e. w. macbride, textbook of embryology, vol. 1, invertebrata (1914); j. w. jenkinson, f. r. lillie, the development of the chick (1919). (e. w. macb.) emigration: see migration. fs encephalitis lethargica.—the virus is unknown. filtrable organisms have been described and also the transmis- sion of the disease by injection of nervous tissues, but these experiments are at present inconclusive. the incubation period is uncertain but it is probably about two weeks. the degree of infectivity must be small. the duration of infectivity and of the persistence of the virus in the body are unknown. apparently the virus may lie quiescent and return to activity after long peri- ods since late manifestations have been known to develop after periods of two to four years, sometimes after apparent cure in the intervals. no age is immune to infection though the disease is not common in very young children. symptoms.—it is very difficult to classify the different cases. the best scheme provisionally is that of macnalty which has been adopted by the ministry of health. this classifies the cases under the following main types:— 1. in which there is general disturbance of the functions of the central nervous system but without localisation. 2. in which, in addition to general disturbance, there are various localisations in the central nervous system. the most frequent of these are affections of the third pair of cranial nerves, but no portion of the nervous system is immune. 3. mild or so-called abortive cases (formes frustes). the clinical manifestations are very variable and complex and are best classified, according to walshe’s scheme into general and nervous. i. general: (1) negative, (2) positive. ii. focal: (1) negative, (2) positive. the positive symptoms denote exaltation of function which may be due either to irritation of nervous tissue or to a loss of the control exerted normally by the higher centres of the brain, while negative symptoms denote depression or loss of function due principally to destruction of nervous tissue. the onset of the disease may be sudden or insidious. occa- sionally the onset is extraordinarily sudden, the patient falling asleep almost without warning or sometimes becoming delirious. more frequently it is insidious either with the development of drowsiness or with more general symptoms. the general symp- toms are not in themselves distinguishable from those of other conditions. headache is common, and nausea and vomiting are frequent without being very severe. the temperature is very variable. it may be about 102° f. at the onset, falling after a few days, or it may be normal at the onset and rise later in the first week. a persistent rise is a serious sign. some cases arc apyrexial throughout. there is no characteristic eruption. in the mild and abortive types the general condition may suggest influenza and the nature of the illness only be recognised long subsequently by the development of characteristic late manifes- encephalitis lethargica tations. most cases undoubtedly commence with some grade of the characteristic symptoms of lethargy and double vision. lethargy.—the depth of this varies from apathy or drowsiness to deep sleep. the patient can usually be roused and may then answer questions with unexpected clearness. occasionally the lethargy gradually deepens to stupor and fatal coma. most commonly the lethargy lasts for two to three weeks, but it varies from a few days to several months. rarely is it completely absent. occasionally there may be somnolence by day and insomnia by night. double viston.—diplopia or double vision also occurs in the majority of cases, and is due to some form of paralysis of the ocular muscles. other ocular manifestations may occur, such as ptosis or drooping of the eye-lids. ‘the pupils may exhibit every possible variation from the normal in size, shape and reactions. coarse nystagmoid movements are also common. optic neuritis is never marked and a definite degree of swelling of the optic disk suggests an error in the diagnosis. nervous symptoms.—certain of the nervous manifestations which occur more commonly during convalescence or after the acute stages ely passed will be referred to under the heading of “ residual” or ‘‘ late manifestations,” but any of them may occur at the ee or at any point in the course of the disease. the positive symptoms are represented by restlessness, deliri- um, acute mania and various degrees of excitement. general convulsions are not common but are usually fatal. negative symptoms are represented by the characteristic lethargy referred to above. not uncommonly a patient may vary rapidly between a positive and a negative state. among the positive symptoms may be mentioned muscular pains which are occasionally severe and may simulate many diseases. in the positive group fall also the rigidity, parkinson- ism and the various involuntary movements which are referred to below among the late manifestations. the negative symptoms are represented by paralysis. the ocular manifestations have been mentioned above. affection of the remaining cranial nerves may produce facial paralysis, difficulty in swallowing, rapid respiration and paralysis of the pharyngeal and laryngeal mus- cles. aphasia and paralysis of the limbs may also occur. the deep reflexes are usually absent in the acute stages of severe cases, but there is no constant rule. the cerebro-spinal fluid may show a moderate increase in the number of lymphocytes but in at least one-third of the cases the fluid is normal. residual and late mantfestations.—these are of the highest importance. the manifestations here referred to develop most frequently during convalescence, yet they may be present from the very outset of the disease, while in other cases long intervals, even of years, may elapse after the acute stages before their ap- pearance. the initial attack may be so mild as to be overlooked at the time. the most important of these manifestations are as follows:— lhe parkinsonian syndrome.—this is characterised by the remarkable absence of facial expression which is termed the “ parkinsonian mask.”’ there is also general muscular rigidity and a striking absence of slight and automatic movements. the general appearance resembles the condition known as paralysis agitans though the general effect is not quite identical. as men- tioned above this state may be fully marked within a week or two of the onset. the progress is more rapid than in senile paral- ysis agitans, but on the other hand recovery may take place, es- pecially in those cases in which it has developed early in the attack. mental changes are not uncommonly associated with this complication especially in childhood. mental changes.—these are of great importance from their frequency and character and are present to some extent in about 75% of cases. the changes may be of all grades from slight weak- ness of the intellectual powers to definite dementia and insanity. the development of certifiable insanity is, however, rare. in adults the usual manifestation is a general weakness of the intellectual powers. the subject shows a marked lack of atten- tion, concentration and initiative. definite melancholia is not very common. there may be some irritability of temper but 979 the excitement which occurs in children is not often seen in adults. insomnia is a not infrequent symptom and may be very persistent. on the other hand there may be persistent drowsiness. in children the mental changes are far more varied and of greater importance than in adults. as in adults the commonest effect is a weakening of the powers of concentration. this be- comes very obvious during school life, since these children pay little attention cither to their work or to their surroundings. such a child makes but little progress and appears to remain at the age at which the illness began, but is seldom troublesome. a second group, numerically smaller, is characterised by ex- citement. ‘these children are usually under 10 years of age. they are restless, troublesome, noisy and uncontrollable. the excitement may be most marked during, or even be confined to the night. as the afternoon passes on these children become rest- less and excited and the entire night is spent in shouting and constant movements. for the time, they are quite beyond control. this group merges into a third group in which there are defi- nite alterations in the moral character. most of the children are between the ages of 10 and 18 years. this group, though small, is important owing to the difficulty in dealing with the subjects. they become morally and sexually perverted. these changes are more common in children of families with a bad record men- tally and criminally. there is a small group in which idiocy de- velops, usually in children under 5 years of age, and in a few cases there has been definite certihable insanity. in addition to these mental changes parkinsonism may be present and also the myo- clonic and other movements described below. the treatment and especially the disposal of children in the second and third groups is a matter of great difficulty. home surroundings are rarely suitable and the child will render ordinary family life im- possible. the moral pervert may also lead to trouble with the law. special institutions may need to be provided for these children. involuntary alovements.—this group of “excitomotor ”’ symp- toms includes numerous varieties of involuntary movements and muscular contractions. though they are more common late in the disease many of them occasionally occur at the onset. the most definite type 1s myoclonus which is characterised by short, rapid, rhythmic contractions of the muscles: a single mus- cle such as the diaphragm, or even a part of a single muscle may be affected. there is generally no movement at the joints, but in some cases the contractions are of a slow rhythmic type and may lead to very complex movements. various choreiform and athetoid movements are also met with. tremors and tics of all types may also develop. epidemic hiccough may be a variety, but this is at present uncertain. respiratory abnormalities —rapid or deep breathing may be present, either continuously or in paroxysms at any period of the acuter stages. violent spasmodic cough occasionally develops in in children or sniffing and various respiratory spasms. paralysis.—the persistence of these is relatively uncommon. diplopia is rarely permanent. many nervous diseases, however, are occasionally simulated such as disseminated sclerosis. nu- merous other late manifestations are observed more rarely. among these may be mentioned obesity which is occasionally associated with polyuria, suggesting disturbance of the pituitary gland. progress and prognosis.—a. j. hall states as a rough approxi- mation that of 100 cases, 25 recover completely, 25 die, and 50 have various residua of which at least 25 exhibit parkinsonism. it is impossible to give a good prognosis during the acute stages in any case of encephalitis lethargica, owing to the fact that serious late manifestations may develop after a mild initial at- tack. further, long quiescent periods may occur with subse- quent recrudescence of activity and fresh manifestations. such intervals may be several years, but the limit is at present un- known. in the acute stages the outlook for life is bad with severe gen- eral symptoms, with a rapid onset, with high temperature, with acute delirium or with mania. slight early symptoms may, how- ever, also progress to serious grades. deep early lethargy is not 980 necessarily a serious symptom. early myoclonus and involun- tary movements do not appear to be especially unfavourable. most deaths occur within the first month and the prognosis for life improves after this. | with regard to special manifestations, the parkinsonian syn- drome may be recovered from partially or even completely when it develops early in the disease, but when it appears later it usu- ally progresses and mental changes may also occur. the outlook in the cases with the severer mental changes is poor; recovery 1s rarely more than partial and relapses may follow. the influence on pregnancy has been carefully studied by hall. he considers that there is no evidence that pregnancy pre-dis- poses to infection with encephalitis lethargica, or that it is associ- ated with a higher mortality or influences the course of the disease in any way. residual manifestations, however, may be ageravated in a woman who has passed through an attack and become pregnant subsequently. no advantage is apparently gained from induction of labour. diagnosis.—the disease is often easily recognised from the combination of lethargy and double vision. the milder forms, however, are easily overlooked at the onset and may frequently be unavoidably mistaken for influenza. when the more complex nervous symptoms are present the diagnosis may be of great difficulty. the conditions with which confusion most often oc- curs are meningitis (especially tuberculous), cerebral tumour, cerebral vascular lesions and cerebral syphilis. the confusion with senile paralysis agitans is not common. treatment.—this is on the general lines of treatment of acute febrile disease. unfortunately there are no specific measures. the administration of hexamine may be recommended. the withdrawal of cerebro-spinal fluid by lumbar puncture is fre- quently performed but there is no evidence that it is beneficial. bibliography.—the monograph on epidemic encephalitis by a. j. hall (john wright & sons, 1924) contains a complete and classified bibliography. the following articles include the earliest reports:— c. von economo, wien. klin. woch., xxx., 581 (1917); aftinch. med. woch., \xvi., 1311 (1919); a. j. hall, lancet, 1., 568, (1918); w. harris, lancet, i., 586, (1918); e. f, buzzard, lancet, i., 616 (1918). (11.1. t.) endocrinology.—this science deals with the structure and function of the organs of internal secretion (endocrine, or in- cretory organs), in health and disease, and has been vigorously developed since ro1o. through observations upon man in private clinics and in university hospitals, and through exper- imentation upon animals in laboratories, many important new facts have been accumulated concerning these organs, their products (imcreta) and their functions during different periods of the life-cycle of individual organisms. attempts are being made to co-ordinate these recent discoveries with what was known before, in order that knowledge in this domain may be better systematised and formulated and that the diagnosis and treatment of endocrine disturbances in man may find securer foundations than the speculative suppositions, often groundless, that have been advanced in a period of expanding research to eke out a store of facts that was felt to be lamentably meagre. limitation of the field.—there is, as yet, no general agreement as to the limits to be set to the field of endocrinology(!). some would include all parts of the body in the category of organs of internal secretion, on the ground that all organs give off cheni- ical substances to the blood and lymph; others would limit mem- bership in the class to a few organs—(a) the ductless glands and (b) certain glands that, despite the possession of ducts and the production of an external secretion, yield to the blood directly (or through the lymph) matcrials that are of profound significance either for the growth and differentiation of distant parts or for the automatic regulation of the physiological functions of those parts. in this latter and more restricted sense, the endocrine organs include the thyroid gland, the parathyroid glands, the hypophysis cerebri (or pituitary gland), the suprarenal glands, the islands of langerhans of the pancreas, and, perhaps, the 1the figures throughout the text refer to the bibliography at end of the article. hn docrinology endocrine portions of the sex gland or gonads. organs, the products of which serve merely the nutrition of the body or are mere waste substances to be excreted from the body by other organs, would not be so included. even with this restriction there 1s doubt as to just how many organs should be classed as “‘ incretory ” and, particularly, as to whether the pineal gland, the posterior lobe of the hypophysis cerebri, the thymus, the liver and the chromaffin tissues should, or should not, be so classed. for practical reasons it would seem better to regard endocrinology, for the present at least, as limited in its field to the structure and functions of the organs of internal secretion in the narrower sense mentioned above rather than to extend its scope so as to include all the organs of the body whose products on entering the blood play any part in chemical regula- tion. the science which deals with the ‘‘ hormones ” or “ chemi- cal messengers ” (cf. e. h. starling) in general within the body has been called ‘‘ hormonology ”’; endocrinology in the narrower sense would appear to represent an important part or domain of that more comprehensive subject. anatomy, histology and embryology of the fendocrine organs. —the gross and microscopic appearances of the principal duct- less glands are described in 8.633. recent advances in the mor- phology of these glands have consisted (2) of intensive studies of their finer structure by means of more delicate histological technique and (5) of more accurate determinations of the mode of origin and the chronological development of these structures in the embryos. systematic (and quantitative) statistical studies of the normal variations of these organs and their relations to the body as a whole have been undertaken for certain animal spe- cies (e.g., the rat). an attempt is also being made to determine precisely the evolutionary processes responsible for the organo- genesis and histogenesis of the structures that manufacture the internal secretions, a phase of research regarded as important by biologists and especially by embryologists(?). general physiology of the endocrine organs.—the increta pro- duced by the endocrine organs and delivered to the blood, either directly, or by way of the lymph, appear to be of great impor- tance for the organism as a whole in all phases of the life-cycle (evolution, maturity and involution). during the period of evolution, the progressive phases of the life-cycle (that is, during the development of the individual), these increta play a rele in the processes of growth, of organ formation and of tissue dif- ferentiation(*). the endocrine substances that participate in the building of the bodily organs and are of significance in the initia- tion and the continuance of the metamorphoses that occur dur- ing growth have been called “‘ harmozones.” it is believed that they react with the autochthonous chromosomal substances of the cells of different regions of the body, perhaps tn an enzyme- like way, and so form a part of the stimulative and assimilative processes concerned in normal growth and differentiation. the endocrine organs themselves undergo changes during growth and development; the several organs appear at some- what different times in the embryo, and vary somewhat in size in structure, and doubtless, too, in function, at different stages of an organism’s unfolding. that the functions of these organs are correlatively connected is certain; and that there are indi- vidual, racial and gencric variations in these correlations seems very probable. the different varieties of habitus (pyknic, as- thenic, athletic) met with inside the limits of normal variation ina single human race, and the skeletal and other somatic peculiar- ities that characterise the different races of man (e.g., nordic, mediterranean, negro, etc.,) or of any other animal, say the dog (e.g., bulldog, dachshund, greyhound), though dependent in part on regional chromosomal specificities, are believed to be also in part determined by the differing endocrine “formulae” or ‘constellations ” of their bearers, the hypophysis being domi- nant, perhaps in one, the thyroid in another, the gonads in a third and so forth. the different combinations and permutations of the incretory influences that are possible are interesting and at the same time appalling to the student of human and animal constitutions). during the period of maturity the hormones from the incre- engine—engineering tory organs are believed to subserve many important functions, particularly those concerned with automatic adjustments within the organism when disturbances endanger equilibrium(®). thus a hormone from the thyroid gland participates in the control of the basal metabolic rate, one from the parathyroid in the control of calcium metabolism, and one from the islands of the pancreas in the control of sugar metabolism. these hormones may act, as is gradually being found out, through the utilisation of various mechanisms; sometimes they exert their influence through affecting other endocrine organs than those by which they are produced, sometimes through the vegetative nervous system and sometimes through direct attack upon the body cells in general. ignorance here is as yet profound. slowly, however, facts are emerging, as a result of clinical-pathological studies, of experimental removal of an endocrine organ and observing what follows, of transplantation experiments and of experiments in which substances derived from endocrine organs are fed to, or injected into, healthy or diseased organisms. during the period of involution of an organism, the incretory organs change as do all the other parts of the body. in the ageing process of normal involution, there may be steady general de- cline through a long series of regressive phases to physiological death. but premature senescence, partial or general, may ap- pear as a result of abnormalities of one or more enclocrine organs (gonads, thyroid, etc.), for such abnormalities lead to disturb- ances of equilibrium of the correlating systems of the body. toa certain extent, rejuvenescence ”’ can be experimentally achieved by injection or transplantation of certain endocrine tissues (sce rejuvenation). it is possible that the appearance of certain malignant growths may come to be correlated with changes in the incretory glands, for in man, alterations in the thymus, thyroid, hypophysis and suprarenals have been found in associa- tion with neoplastic growths and, in experimental animals, defi- nite influences of incretory conditions upon the “ taking ” and growth of neoplastic grafts, have been demonstrated(®). adeno- carcinomata, for example, are transplantable to certain normal mice but not to castrates of the same species, though if a young testis be also grafted into the castrate, the neoplastic graft can then be made to grow. physiological chemists have made progress in isolating various potent chemical substances from the organs of internal secretion. among the more important of these may be mentioned epi- nephrin (or adrenalin) from chromaffin tissues, t/yroxts from the thyroid gland, pituitrin and antuttrin from the hypophysis cerebri, /utein from corpora lutea of the ovary and insulin (g.v.) from the islets of the pancreas. recently, from the parathyroid glands has been extracted a parathyroid hormone that will pre- vent or control tetany due to parathyroid insufficiency and will regulate the level of the calcium content of the blood; from pitu- itrin has been isolated an active principle of great potency that will in extremely dilute solutions cause contraction of uterine muscle; and from ovarian follicular fluid has been separated a hor- mone that will, on injection, induce a sexually mature condition in immature females(? ve clinical endocrinology —knowledge of clinical syndromes due to disorders of the endocrine organs has also made rapid strides since 1910, and the symptomatology of the endocrine disorders has been extended and made more precise. thus the principal endocrine disturbances manifested as graves’ disease, myxoe- dema, tetany, acromegaly, adiposogenital dystrophy, eunuchism and eunuchoidism, addison’s disease, suprarenal virilism and hirsutism and diabetes mellitus are now far better understood than ever before. there has been a tendency, however, to at- tempt to push the clinical applications of the scientific advances in endocrinology further than is warranted, and much confusion has arisen both in the profession and among laymen because of failure to distinguish between mere speculations and well-estab- lished facts(§). fortunately, warnings have been sounded, and more careful workers emphasise the necessity of a more rigidly critical approach to the solution of clinical endocrine problems. this is particularly true of endocrine therapy,which has recently almost run riot. the wishful thinking of unskilled practitioners qosi and their uncontrolled impulses therapeutically to act by ad- ministering endocrine products singly and in the form of div- ers ‘‘ polyglandular formulae’ bade fair, for a time, to become scandalous, aided and abetted as such ill-founded therapy was by ignorant or unscrupulous vendors of endocrine products. organotherapy and hormonotherapy have scored genuine tri- umphs; it is unfortunate that their reputation should be sullied by uncritical extravagances(*). the effects of thyroid extract or thyroxin in myxcedema, of iodine in the prevention of colloid goitre (q.v.), of surgical removal of part of the thyroid gland in exophthalmic goitre of insulin (¢.v.) in diabetes (¢.v.) are brilliant examples of therapeutic successes in endocrine domains. the utilisation of certain hormones for their pharmacodynamic effects in disorders not primarily of endocrine origin is proving profitable; the control of paroxysms of bronchial asthma by in- jections of epinephrin and the restriction of water-excretion through the kidneys in diabetes insipidus by injections of pitu- itrin, and the use of pituitrin in obstretrics and surgery, of adrenalin in local anaesthesia and of lutein in ovarian dysfunc- tion are notable examples. organotherapy and hormonotherapy undoubtedly have an important future, but genuine success in these fields can be achieved only by thoughtful, painstaking, rigidly controlled work; progress ts only too likely to be retarded and science to be discredited by the rash enthusiast and the credulous ignoramus. in the united states the assn. for the study of internal se- cretions and its journal, endocrinology, are doing much to foster the development of endocrine research. bibliography.—for summaries of the present status of clinical endocrinology, the reader may consult: endocrinology and metabol- isi, 5 vol. (1922), ed. by l. f. barker; a. biedl, jnnere sekretion (berlin, 1922); w. falta, the endocrine diseases (y ork, pa., 1923); m. e. gley, les secretions internes (1914); r. porak, les syndremes eendocriniens (1924); w. timme, lectures on endocrinology (1924); a. weil, the internal secretions (ard ed., 1924); and h. zondek, die krankheiten der endocrinen driisen (1923 e (1) cof. a. kohn ueber den baie. sekretion, med. klinik, berl.,. 1924, 20,.1272-1274. (2 for summaries of the morphological and developmental dis- coveries of recent years, the reader may consult ic. a. schafer’s endocrine organs; an introduction to the study of internal secretion (1916). swale vincent’s zuternal secretion and the.ductless glands (2nd ed., 1922), and the series of articles in the first two vol. of endocrinology and metzbolism (1922). (3) ef. harms ‘' das wesen der inkretion und ihre bedeutung fiir das normale und experimentell beeinflusste geschehen innerhalb der lebensphasen der tiere,” deutsche med. wehnschr., 1925, 51, 631~ 63 3 er cf. g, draper, /tuman constitution: a consideration of its rela- tion to disease (1924); also, j. bauer, die konstitutionelle disposition su inneren krankheiten (3. autl, 1924). (5) cf. w. b. cannon and associates “studies on conditions of activity in endocrine glands,” ai. jour. phystol., 1924, 69, 46; 71, 153; 1925, 72, 283; 295; also gn, stewart, physiol. rev., 1924, 4, 163-190, (6) cf. brown and pearce “ malignant tumor of rabbit: results of miscellaneous methods of transplantation, with discussion of factors influencing transplantation in general.” j.» exper. afed., 1923, 37, 811; 38, 385, 1924, 40, 603. (7) for references to some of these newer studies, the reader may consult a. c, crawford’s ‘* chemistry of the suprarenal glands, endocrinology and metabolism oie): 2, 77-98; e. c. kendall's ‘ isolation of thyroxin,"’ j. am, m. assn., 1915, 64, 2042- 2043; j. j. abel’s “ physiological, chemical and clinical studies on pituitary principles,’’ bull., johns hopkins hosp., 1924, 35, 305-328; f. g. banting and c. h. best’s “internal secretion of the pancreas (insulin)” j. lab. & clin, med., 1922, 7, 251-464; j. j. r. mcleod and f. g. banting’s “ antidiabetic functions of the pancreas and the successful isolation of the antidiabetic hormone—insulin,” (1924); e. allen and e. a. doisy’s ‘* ovarian follicular hormone,” am. j. physiol., 1924, 69, 577-588; j. biol. chem., 1924, 61, 711-7273 and j. b. collip’s ‘ parathyroid hormone,”’ j. ‘biol. chem., 1925, 63, 395-438. (8) cf. h. a. christian ‘‘ the use and abuse of endocrinology,” canada m, assn. j., 1924, 14, 102-106, (9) cf. h. lisser, ‘ organotherapy; present achievements and future prospects,” endocrinology, 1925, 9, 1-20. l. f. b.) engine: see internal combustion engine; steam engine. engineering: sce bread making; cellulose; chemical engineering; dams; dynamo; electricity; factory design; ferro-concrete engineering; fuel problems; gas, manu- 982 facture of; hydroelectric engineering; internal combus- tion engines; illuminating engineering; irrigation engi- neering; materials, strength of; papermaking; pumps; research, industrial; river and canal engineering; steam engine; textile machinery; turbines, steam.