1897 Human Embryology 4

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Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Human Embryology: Introduction | The Uterus | General Outline of Human Development | The Genital Products | History of the Genoblasts and the Theory of Sex | The Germ-Layers | Segmentation | Primitive Streak | Mesoderm and the Coelom | Germ-Layers General Remarks | The Embryo | The Medullary Groove, Notochord and Neurenteric Canals | Coelom Divisions; Mesenchyma Origin | Blood, Blood-Vessels and Heart Origin | Urogenital System Origin | The Archenteron and the Gill Clefts | Germinal Area, the Embryo and its Appendages | The Foetal Appendages | Chorion | Amnion and Proamnion | The Yolk Sack, Allantois and Umbilical Cord | Placenta | The Foetus | Growth and External Development Embryo and Foetus | Mesenchymal Tissues | Skeleton and Limbs | Muscular System | Splanchnocoele and Diaphragm | Urogenital System | Transformations of the Heart and Blood-Vessels | The Epidermal System | Mouth Cavity and Face | The Nervous System | Sense Organs | Entodermal Canal | Figures | References | Embryology History
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Chapter IV. Segmentation Formation of the Diaderm

There follows after impregnation a short pause, and then the ovum begins its process of repeated division, which is known as the segmentation of the ovum," the term having been introduced before it was known that each " segment" is a cell. The division or cleavage (Furclnuig) of ova was described by Prevost and Dumas, 1824, and again by Rusconi, 36.1. By usage the term segmentation is restricted to the production of cells up to the period of development when the two primitive germ-layers are clearly differentiate and the first trace of organs is beginning to appear.

Segmentation Nucleus

The impregnated ovum has a single nucleus, which is known as the segmentation nucleus, and which is formed, as stated in Chapter III., by the union of the male and female pronuclei.* It is the parent of all the nuclei subsequently found in the organism, and participates actively in the process of segmentation. It is very much smaller than the nucleus of the eggcell before maturation ; it is usually membranate and has numerous fine granules of chromatin, niicrosoma, derived from the pronuclei ; in some cases the microsoma from the male pronucleus are distinguishable from those of female pronucleus (see under Impregnation, ante, p. 70). In the rabbit the nucleus when first formea has indistinct contours, irregular shape, and a homogeneous appearance (Eld. van Beneden, 75. 1, 009) ; it soon enlarges, becomes regular, and acquires a distinct centrally situated nucleolus (Bischoff, 42.1, 50, Coste, 47.1, Lapin, PI. II., Fig. 4), presumably by the gathering together of the microsoma.

The position of the nucleus is always eccentric, so far as known, and aproximately, if not exactly, the same as that of the egg-cell nucleus before maturation. Accordingly, the degree of eccentricity varies as the amount of yolk or deutoplasm, being least in alecithal and greatest in telolecithal ova. In brief, it may be said the nucleus tends to take the most central position possible with regard to the protoplasm of the ovum. The vitelline granules are not to be regarded as protoplasm, hence their accumulation may produce a onesided distension, without, however, in the least disturbing the uniform radial distribution of the protoplasm. The nucleus is surrounded by protoplasm with few or no yolk-grains ; in telolecithal ova the perinuclear accumulation is the court of protoplasm at the animal pole.

  • fid. van Bcoeden id hi«« flrst papor oo ARoaris. 83.1. afflnned that there was no real uoion of the pronuclei In the Inipn'jrnat*^! ova of that K]KHrle«: but Camoj-, 80. 1. shows that Van Beneden *s olieen'at ions were incomplete, and Zacharlas has stattnl. K<.1. that they are so de« fective as to t^e fundamentally prruncouH in n*)ranl to imitortant phaseK. and he points out that in realilv theef?^ of Ascaris oflr»*r another pnH)f of the actual union of the pronuclei. Thelmpreg^atloD in tliis neniatcKl haH sincv formed the Kuhlect of numenms articles; see Van Beneden and Nevt H7.1. Camov S7.1, Boveri KK.l. O. Hertwijr IW 1. etc.
  • It Is often Ktate<I that (he nucleus 'ieH *-xartht in the centre, but 1 have been unable to And a •ingle observation to justify the statement.

Period of Repose. — After the segmentation-nucleus is formed there occurs a pause, which lasts, according to observations on several invertebrates, from half to three-quarters of an hour. It is probable that a similar pause ensues in the mammalian ovum, but there are as yet no observations to show whether it occurs or not. During this period the yolk expands slightly, unless, indeed, the expansion observ^ed is due to the influence of hardening agents,* and the monocentric radiation, which is present when the nuclei copulate, gradually fades out, and is replaced by a dicentric radiation, which marks the end of the period of repose and the commencement of the first division of the oviun.

Karyokinesis of the Ovum. — For convenience I interpolate a sketch of the process of cell-division as encountered in the ovum, based on O. Hertwig, 88.1, 37, and C. Rabl, 84.1. My sketch is by no means complete.

It is probable that the resting nucleus has one pole at which the connection l)etween the reticulum of the nucleus and the surrounding protoplasm is more intimate than elsewhere, as Suggested by Rabl, o8.1. This polo is marked by a clearer sjwt outside the nucleus, close against it, and much smaller than it. This clear spot becomes the centre of a radiating arrangement of the protoplasm. It was, I believe, first observed by Flemming in the eggs of Echinoderms, has been seen in Ascaris megalocephala by Van Beneden and Neyt, 87. 1, and by Boveri, 88. 1, in Siredon by Kolliker, 89. 1, and in other cases. It is now designated as the sphere of attraction, t and is seen, at least in certain phases, to contain a separate central body (centrosoma of Boveri) . It is not improbable that the *' sphere of attraction" is identical with the Xehenkern of recent German writers. In a number of instances a small part of the nucleus is seen to separate off and to lie as a distinct body, Nebenkem, alongside the nucleus ; this body has a colorable portion, which is comparable to the "centrosoma." For an account of the scattered observations on the Nebenkem, together with the relation of these bodies to Gaule's so-called cytozoa, see G. Platner, 86.3. For additional observations see Prenant, 88.1, and Platner, 88.2. The sphere of attraction divides, as does also its central body, and its two parts move to opposite sides of the nucleus. There thus appear two opposite accumulations of clear protoplasm, from each of which as a centre astral rays or radiating lines are formed in the cell-l)ody. Meanwhile within the nucleus changes go on; the threads of the intranuclear network radiate out from the pole, where the sphere of attraction lies before its division, and the chromatic substance forms a number of distinct grains. When the sphere of attraction divides and its halves go asunder the nuclear substance preserves its radiating relation to each sphere, and as the membrane of the nucleus disapi)ears during these changes the final result is the transformation of the nucleus into a spindle-shai)ed Ixxiy, the ix)ints of wliich rest just within the clear centre of eacii astral system, so that the spindle stretches from one protoplasmic mass to the other. The spindle consists of fine threads extending from pole to pole and having almost no affinity for the dyes of the histologist — a peculiarity which causes them to be known as the achromatic threads. These threads are probably always compoimded of a considerable number of exceedingly fine fibrilis (see Rabl, 89.1,21,2*2) The colorable su Instance forms a number of separate grains, each of which is unite<l with one of the achromatic threads, and all of which lie at the same level in the centre of the spindle; when the spindle is seen from the side, the chromatine grains appear to constitute a central band or disc (Strassburger's Kenipkiite)^ but when the spindle is seen endwise the separate grains are at once recognized. The shape of the grains is variable; some authors without sufficient observational proof have advanced the opinion that the grains are always V-shaped. The spindle, together with the polar accumulations of protoplasm and the two accompanying radiations, constitute a so-called amphiaster.

  • Van Beneden states that arsenic acid prodiic<»« an artificial expansion of the ovum within thexona. ^

t The history and sijrnifloance of the sphenw of attraction, as here presented, cannot by imy means be rejs:arde<l as final. The ol»servations are few. and until recently the exact history of the spheres of attraction iius i-eceived no attention from investigators.

The domain of the radiation extends, the two protoplasmatic centres move farther apart, the nuclear spindle elongates correspondingly, and the chromatin grains of the Kcrnplatfe divide. Flemming maintains that the division is always lengthwise of the V-shaped grain, but this has been controverted by Canioy. How the division occurs in the mammalian ovum is unknown. 3y tho division, however it is effected, the number of chromatin grains is doubled; they form two sets: one set moves toward one ix)le, the other toward the other pole; the grains of each set keep at the same level as they move imtil they reach the end of the S])indle, where they apix^ar as a polar disc (Carnoy's couronne j^olaire). Next the achromatic threads of the spindle break through and are apparently drawn in toward each polar crown. There are now two nuclear masses, each near, but not at, the centre of a radiation, and each consisting of chromatin and achromatic substance. Each mass develops into a complete membranate nucleus, but the steps of this process have yet to be followed in detail in the vertebrate ovum.

The signs of division of the protoplasm usually become visible about the time the i)olar crowns are formed, but when the ovum contains much deutoplasm the division may be retarded. In the plane which passes through the equator of the nuclear spindle there appears a furrow on the surface of the ovimi, which gradualh' spreads and deepens until it is a complete fissure around the cell ; it cuts in deeper until at last only a thin stalk connects the two halves of the cell, and thereupon the stalk breaks and the cell is di\'ided. There next ensues a pause, during which the astral rays of the protoplasm disap}^ar in the daughter-cells, and the daughter-nuclei assume each the form of an ordinary resting membranate nucleus.

The (external appearances of segmentation in the living ovum vary, of course, especially according to the amount and distribution of the yolk-material. The appearances in holoblastic ova with very little yolk are well exemplified by Limax campestris. Mark's description, 81.1, is, nearly in his own words, as follows: In Limax, after impregnation, the region of the segmentation nucleus remains more clear, but all that can be distinguished is a more or less circular, ill-defiiied area, which is less opaque than the surrounding portions of the vi tell us. After a few moments this area grows less distinct. It finally appears elongated. Very soon this lengthening results in two light spots, which are inconspicuous at first, but which increase in size and distinctness, and presently become oval. If the outline of the egg be carefully watched, it is now seen to lengthen gradually in a direction corresponding to the line which joins the spots. As the latter enlarge the lengtbeniug of the ovum increases, though not very conspicuously. Soon a slight fattening of the surface appears just under the polar globules ; the flattening changes to a depression, Fig. 40, which grows dee|>er and becomes angular. A little later the furrow is seen to have extendeti around on the sides of the yolk as a shallow ^jKjj. ^^ depression, reaching something more than half way toward the vegetable or inferior pole, and in four or five minutes after its appearance the depression extends completely around the yolk. This annular constriction now deepens on all sides, but most rapidly at the animal pole : as it deepens it becomes narrower, almost a fissure. Bj- the further deepening of the constriction on all sides there are formed two equal oiii^riadurinBth;.n;M masses Connected by only a slender thread of ciMVBge. The enwioiwB protoplasm, situatcd nearer the v^tetative than L^Mark ™" ajodiaiim' the animal pole, and which soon l)ecomos more attenuated and finally parts. The first cleavage is now accomplished. Both segments undergo changes of form; they approach and flatten out against each other, and after a certain time themselves divide.

Primitive Type of Segmentation

In the lower animals there is not found that excessive amount of deutoplasm in the ovum which is so characteristic of the vertebrates, and in their ova we have what is undoubtedly the earlier and more primitive type of H^mentation. In these cases the cleavage extends, as in the egg of Limax (see above), through the whole of the dividing- cell. The two cells first produced are almost if not (juite alike, and each of them produces two cells which are also very similar to one another; then comes a division of the four cells into eight, four of which resemble one another and differ from the remaining cells which are also simiUr among themselves. Four of the cells are derived chiefly from the substance of the animal pole of the ovum and are very protoplasmatic ; and the other four cells are constituted out of the substance of the v^etable pole and accordingly contain most of the deutoplasm of the ovum The eight cells form an irregular spheroid, in the centre of which there is a space between the cells ; this space is known as the segmentation cavity.

The four cells of the animal pole progress in their divisions more rapidly than the four of the v^tetable pole; but the latter, when the yolk matter is at a minimum, as, for instance, in echinoderms, do not lag much From their unequal rates of division the two sets of cells come to differ more and more in siise, those of the animal pole being much tho smaller. The divisions of the cells take place so that the ceils form a continuous layer of epithelium, one cell thick, stretching around the enlarged central segmentation cavity, Figs. 47 and 60; the epithelium consists of a larger area of the small cells of the animal pole and a small area of the largo cells of the vegetable pole. This stage of segmentation is known as the blast ul a stage ; the small cells are destined to form the ectoderm of the embryo ; tho large cells the entoderm, the central space is the segmentation cavittf; tho line along which the two parts of the epithelium (ectodenn and entoderm) join is known as tho evtental line X^^52SCCi»f\yEn

Vertebrate Type of Segmentation

In the vertebrates we find that segmentation nio«f»io o*-^ ^r also results in two epithelia, an ectoderm and Echinooardium cordatum, entoderm, joined at their edges, and surround- *^!^°2S^'rr^;'"Sf°*eoto: ing a segmentation cavity, but the resemblance ^^J^^' After ^^Sfi*^**^** to the tj'pical blastula is marked by changes in both ectoderm and entcxlerm; the vertebrate ectoderm when first fully differentiateil consists of several layers of cells, and not merely of a single layer of cells, as in the primitive type of segmentation ; the entoderm contains a very large amount of nutritive material (deutoplasm) , and is represented either by a large mass of large cells (marsip^^branchs, ganoids, amphibians) or a mass of protoplasm, not divideil into cells or but partially divided into cells, and containing an enormous quantity of deutoplasm (sauropsidans and monotremes) . In the higher mammals there are further modifications, described below.

The more primitive form among vertebrates is, I think, presumably that in which the entoderm consists of separate cells; for this mode of segmentation is the one which most resembles that of invertebrates, and it occurs in the lowest vertebrates, and in ova which are not excessively charged with yolk.

In the primitive form of vertebrate segmentation^ which is preserved in the marsipobranchs, ganoids, and amphibia, there is a well-marked difference between the cells of the two poles. The following account refers especially to the frog's egg and is an adaptation of Balfour's summary ('* Comp. Embryol.," I. , 78, 70) . The first formed furrow is vertical , it commences m the upper half of the ovum, which correspi^nds to the animal pole, and is characterized by the black pigment — the lower or vegetable pole being whitish. The first furrow extends rapidly through the upper, then more slowly through the lower half of the ovum, so that the divergence in the two polar rates of development is indicated alread}'. As soon as the tunx)w has cleft the egg into halves, a second vertical furrow appears at right angles to the first and behaves in the same way. Fig 48. The next furrow is at right angles to both its predecessors, and therefore parallel to the equator of the egg; but it is much nearer the animal than the vegetative pole. It extends rapidly around the e^ and divides each of the four previous segments into two parts: one larger with a great deal of yolk and the other smaller with very little yolk. The eight segments or cells have a small segmentation cavity in the centre between them. This cavity increases in size in subsequent stages, its roof being formed by the small cells further divided, and its floor by the large cells also multiplied by division, though to a less extent than the small cells. All the developmental processes progress more rapidly at the animal pole. After the equatorial furrow there follow two vertical or meridional furrows, which begin at the animal pole and divide each of its foiu* cells into two, making eight small cells. After a short period these furrows extend to the lower pole and divide each of the large cells into two, Fig. 48, 4- The so-called meridional cleavages after the first and second are not true meridional cleavages, since they do not pass through the folds of the ovum, but through the poles of the cells (blastomeres) , which they divide (see Rauber, Morph. Jahrh., VIII. , 287). A pause now ensues, after which the eight upper cells become divided by a furrow parallel to the equator, and somewhat later a similar furrow divides the eight lower segments. Each of the small cells is now again divided by a vertical furrow, which later divides also the corresponding large cell. The segmentation cavity is, therefore, now bounded by 32 small and 32 large cells. After this the upper cells (ectoderm) gain more and more in number beyond the lower cells (entoderm) . After the 04 segments are formed two equatorial furrows appear in the upper pole before a fresh furrow arises in the lower, making 128 ectodermal cells against only 32 entodermal. The regularity of the cleavage cannot be followed further, but the upper \}o\e continues to undergo a more rapid segmentation than the lower. At the close of segmentation the egg forms a sphere containing an eccentric segmentation cavity. Fig. 49, ,s*. c, composed of two unequal parts, an upper arch of several layers of cells, /?/, the primitive blastoderm of Minot or ectoderm, and a lower mass. Yolk, of large cells rich in protoplasm. At the edge of the mass of large cells, A'?r, there is a gradual passage in size to the cells of the blastoderm, and it appears that the small cells receive additions at the expense of the large ones; this zone corresponds to the so-called germinal wall of large vertebrate ova, and also to what we liave defined as the ectental line.

Fig 4H.— Segmentation of the ep^: of tli«M'oium<»n Frojr

The secottdary type of vertebrate segmentation differs from the primary principally in the retarded development of theent<xlerm, due apparently to the increiise of the yolk-matter. The yolk-granules are, as already mentioDed, found to be situated not quit© exclusively, though lUmost mi, in those parts of the o\'uiii out of which the eutodermal cells are formed. Hence, when there is a great deal of yolk the anla^ of the entoderm becomes bulky, and when it segments the entodermal cells it produces are correspondingly hig. as we have seen is the case in amphibian ova. ( )n the other hand, when the amount of yolk is small, a.1 in the primitive type of segmentation, e.g. cchinotiennH, the cniodermal cells are small . In the reverse case, when the amount of yolk is exceedingly great, as in selachians, reptiles, and birds, the yolk may not divide into cells as fast as the nuclei multiply, so that it eeenis that the presence of ihe deutoplasm, though it does not affect the nuclear divisions markedly, certainly , impedes very much the di- Jj^^'g,,,'!! vision of the protoplasm, and conse<juentIy in these ova we find, at ceitain stages of development, a multinucleate yolk. The imiiedinierit is not encountered by the jirotoplasm of the animal jxile, hence we see the animal pole segmenting while the yolk does nttt : in this case the segmentation apI)ear3 confineil to one iwrtion of the ovum, and, accordingly, such ova are termed merohlo-itic in contradistinction to the holoblastic ova, in which the first cleavage furrow divides the whole ovum ; but the difference, it must be expressly remembered, is one of degree, not of kind.

The best known example of a vertebrate meroblastic ovum is undoubtotily the hon's e^:^. The so-calletl yolk, or "yellow," is the ovum ; the white and the shell are botli adventitious envelopes added by the oviduct as the ovum passes down after leaving the ovarj'. The s^mentation begins while the o\-um is passing through the lower part of the oviduct, and shortly l>eforo the formation of the shell commences. If an ovum from the upper part of the oviduct be examined it is found to be surrounded with more or less white (albumen). Its animal jwle is represented by a whitish disc from 2.5-3.5 mm., in diameter, and O.-IO-O.;!,! mm. in thickness; this disc is known by many names; Formative yolk, germinal disc, cicatricula (Narbe, Hahnentritt, Keimscheibe, stratum s. discus proligerus). The animal jwle consists chiefly of protoplasm, and is peculiar only in its small size compared with the whole ovum, it contains, when the ovum leaves the ovary, the egg-cell nucleus; the o^iim then matures, impregnation occurs, and finally segmentation begins. Viewing the i>vum from above we see the first furrow appear as a groove running across the germinal disc, though not for its whole width, and dividing it intu) halves ; this furrow is developed in accompaniment with the diyiision of the segmentation nucleus. The primary furrow is succeeded by a second fumiw nearh" at right angles to the first ; the surface of the germinal disc is cut up into four segments or quadrants, Fig. .'>ii, A, which are not, however, separated from the underlying substance. The number of radiating furrows increases from four to seven or nine, when there arises a series of irregular cross-furrows, by which the central portion of each segment is cut off from the peripheral portion, giving rise to the appearance illustrated by Fig. 5(i, C; there are now a number of small central segments surrounded by larger wedge-shaped external sej . ments. Division of the segments now proceetls rapidly by means ol furrows running in various directions. !Not only are the small central segments divided into still smaller ones, D, but their uum ber i» increased also by the addition of cells cleft off from the central ends of the lai^ peripheral segments, which are themselves subdivided by additional radiating furrows, Sections of the hanlened germinal disc show that segmentation is not confined to the surface, but extends through the protoplasmatic mass of the animal pole, there being deep-seated cleavages in planes parallel to the surface, of the ovum. According to Duval, 84. 1, when the first few small central cells are separated off, there is a small space between them and the underlying egg-substance (see Figs. 2, 3, 4, 5, and G of his PI. I.), and this space he calls the segmentation cavit}' ; but in this I think he is in error, for the cells formed below this space are incorporated in the ectoderm or primitive blastoderm; the cells referred to are those marked in in Fig. 8 of Duval's PI. I. The true segmentation -cavity, as we have seen, is bounded on one side by ectoderm, on the other side by entoderm. This fundamental characteristic Duval has entirely overlooked. From the processes described there results a disc of cells, which receives peripheral additions ; the border from which these additions come is known as the segmenting zone. The whole mass of cells derived from the germinal disc represents the ectoderm, and the segmenting zone may be homologized with the cells around the edge of the primitive blastoderm of the frog. Fig. 40, Avr. A section through the segmented germinal disc shows the following relations : The blastoderm is a disc of cells ; its upper layer is epithelioid ; its lower layers consist of rounded cells more or less irregularly disposed; at its edge it merges into the yolk, which continues to produce cells ; between the blastoderm and the yolk is a fissure, the segmentation cavity ; the yolk under the fissure contains a few nuclei, which have each a little protoplasm about them, but do not form parts of discrete cells.

In reptiles the process of segmentation is very similar to that in birds. Our knowledge is based principally upon observations upon the eggs of the European lizards (Lacerta agilis and viridis), which have been studied by Itupffer and Benecke,78.2, Balfour, 79. 1, Sarasin, 83.1, Weldon, 83.1, and Hofmann {Archives neerlandaises^ XVI., 1881). Hofmann gives a resume in Brown's "Thierreich," VI.,Abth. III., pp. 1877-1881. The process is very irregular, forsmall cells are budded off singly and in scattered clusters from the larger segments. As Strahl, 87.1, 290, has pointed out, the blastoderm receives direct accretions from the underlying yolk, cells being separated off by horizontal cleavages. At the close of segmentation the germinal disc is converted into a membrane consisting of several layers of cells and parted from the underlying yolk by a thin space, the segmentation cavity ; at its edge this membrane, the primitive blastoderm, is united with the yolk, it being immediately surrounded by a segmenting zone, from which it receives accri&tions. The layer of the yolk immediately under the segmentation cavity contains scattered nuclei, hnng singly or in clusters; each nucleus is surrounded by protoplasm; the nuclei are not all alike; some are i;erz/ large, round with very distinct nuclear threads ; other are small and often bizarre in shape ; probably the latter are budded off from the former.

In Elasmobranchs the germinal'disc is thicker, and consequently the mass of cells resulting from its segmentation cuts in quite deeply into the yolk (Balfour, Comp. Embr>^ol," I., Fig. 40; Ruckert, 85. 1, 28). Kastschenko, 88.2, has shown that before the germinal disc is segmented into cells there are nuclei scattered through it, and he has rendered it probable, 88. 1, that these nuclei come from the segmentation nucleus. It is possible that in other meroblastic vertebrates proliferation of the nuclei precedes the cleavage of the germinal disc into discrete cells. As segmentation progresses, the cells spread out into a layer which shows the same essential relations as have been described in birds and reptiles. There is the several-layered primitive blastoderm, with its edges connected with the yolk and itself overlying the segmentation cavitj', the lower floor of which is formed by the multinucleate yolk, the representative of the cellular yolk-mass of the frog, Fig. 40, Yolk. The nuclei are confined to the layer immediately under the segmentation cavity, and this layer corresponds to the sub-germinal plate in teleost ova. Of the yolknuclei some are large, others are small as in reptiles ; they are the Parablastkeme of His, the Meroc^iienkeme of Riickert.

In bony fishes also we find the same type, but modified somewhat. The process of segmentation has been very carefully studied by C. O. Whitman, 84. 1, to whom I am indebted for the iU'companying semidiagrammatic figure of the segmented ovum of a flounder. The ovum is surrounded by a vitelline membrane, z, from which it has slightly withdrawn, notably at the upper jx)le, where lies the thick cap of cells constituting the blastoderm, Bl; in the stage represented the outer layer of cells is just beginning to assume an epithelioid character; undernejith the blastoderm is the well-marked segmentation cavity, s. c. ; everywhere at the edge of the blastoderm lit^s the segmenting zone, k /c, a ring of granular protoplasm with rapidlydividing nuclei; the cells resulting from these divisions are added to the edge of the blastoderm, which thus enlarges i)eripherally. The protoi)lasm of the segmenting zone is prolonged inward, forming the fl(K>r of the segmentation cavity; this sheet of protoplasm, ,s.(/., is known as the sub-qerminal phftf*. The segmenting zone is, of course, the homologue of the similar zone in amniote ova, ^ or the so-called germinal wall, ' but it is quite sharply defined against the yolk, and therein differs from the wall in the chick, because in the latter the germinal wall merges graluallv into the yolk. The process of segmentation differs from that in elasmobranchs and sauropsida in that the cleavage of the germinal disc is strikingly regular, and further in that the whole width and thickness of the germinal disc is involved in the segmentation from the very start. The segmentation in teleosts is further interesting as affording proof that all the nuclei, as shown by Whitman's investigations, arise from the segpiientation nucleus.

Fig. ril— Ovum of a FloinhkT in transverse* vertical section; senii-dia^rnmniatic fljfurv by Dr. C. O. WhituiAu. £, vitelline membrane (or zona?); kic, sefnnentinfic zone (Keimwnih; HI, bhtstoderm or primitive ectoderm: m.c, Bej^mentation cavity; ».(/., sub^rminul plate; gl, oil globule of yolk.

To auinmarize : Invertebrate ovn with a lai^ yolk, which does not divide into cells until segmentation is conaideralily ailvance<l, the substance of the animal \>o\e segments completely, and produces several layers of cells (the uppermost becoming epithelioid) which are the ecto<lenn or primitive blastoderm ; the edge of tho blastoderm touches the yolk, and is surrounded by a nucleated zone in which the production of cells is continuing; underneath the blastoderm is the fissure-like segmentation cavity; the floor of this cavity is formed hy the unsegmentated yolk (entoderm) which is furnished with scattered nuclei in the layer immediately underneath the yolk ; the yolk nuclei, at least in selachiiuis and reptiles, are of two kinds, very large ones and smaller ones, which arise probably from the large nuclei ; the nninucleated layer may lie termed the sub-germinal plate.

Modified Segmentation of Placental MammalB.— The lowest mammals resemble the reptiles in many respects. Among other reptilian characteristics of the monotremes wo find ova of largo size and rich in deutoplasm. That these i-va segment in similar manner to those of reptiles and during their pasMage through the oviduct was first ascertained liy direct observation bv Caldwell in I8M, 87.1.

Ill marsupials and the placental mammalia tho amount of yolk-substance U greatly reduced, and the ovum is of small size. It is, therefore, holoblastic.

that is to say, the cleavage planes cut through tho entire cell, as in the primitive type of segmentation; but the fo^r ThrBS arrangement of tho cells at the close of jX3i;?S,^ra'hSi?1h;«rtaHS

S^nientation appears to l»e a direct in- Ih*- polar Blohiilnc niimerouB Bperma heritance from the ivptiliim ancestors iioU* "^ " ""' mntui ^onape uof the mammals.

The segmentation of the mammalian o\iim was first clearly recognized by Biscboff, though it had l>een previously seen and misinterpreted by Barry, 38.1, 39.1, 40.1; very bejiutiful figures of segmentation in the rabbit have Ijeen given by Coste, 47.1. More recently obser\"ations have l>cen published by Heiisen on the rabbit, 76'. 1, Van Beneden on the rabbit, 76.1, 80.1, KupfFer on rodents, 8.33, Selenka on nxlents, 83.1, 83.1, 84,1, and opossums, 86. 1, Van Beneden and Julin on bats, 80. 1, Tafaiii on white mice, 89.1. The ommwhen dischargetl from the ovary is surrounded by the corona radiatti (r/. ante, p. 5'.*)^ which is lost when impregnation takes place. Segmentation begins when the ovum is one-half to two-thirds of the way through the oviduct. The o%'um spends about seventy hours in the oviduct in the rabbit and about eight days in the dc^. the first cleavage plane passes through the axis of the ovum, which is marked by the jwlar globules. When first formed the two si"gmentation spheres are oval and entirely separated from one another, but subsequently they flatten against one another and become Hppressed — a remarkable phenomenon, of which no explanation whatever. The second cleavage plane is alsii meridional.

The ovum next divides into eight and then into twelve s^mentn, of which four are latter than the rest.

The succeeding cleavages have never been followed acourately ; but from Heape's observations on the mole, 86.1, HIO, we know that the divisions progress with great irregularitj-, and it is probable that the commonly assumed regularitj- of mammalian segmentation does not exist in nature. After a time (in the rabbit about seventy hours) there is reached the stage termed Meiagastrula by Van Eeneden, 80.1, 153-160, in accordance with his view of the homologies of this stage. The metagastrula consists of a single layer of cuboidal hyaline cells lying close against the zona pellucida, Fig. fi-!, en; the space within tills layer contains an inner mass of cells, im, which are rounded or polygonal and densely granular. At one ix>int the outer layer is interrupted and the space is filled by one of the granular segments of the inner mass, Fig. 5n. The nuclei of all the cells are somewhat nodulateil and liave several highly refractile granules each. The granules in the bodio.'i of the cells of the ouUt layer are somewliat con(*ntrated around the nucleuw, Jr-Uo. leaving the cortices of thi' cells clear. Van Beneden, 78.1, 28, 2U, has observ-wl that sometimes {'H ova out of 20) the first two segmentation spheres are of unequal size in the rabbit, an<l similar van a bilitv occurs in the mole, Heaije. 86.1, ICi; Tafani, on the other hand, expressly denies itsoccurrencein white mice. It is, I think, very improbable that this difference, which sometimes occurs and sometimes does not, has any fundamental significance. Van Beneden, however, lias maintaine<l that the small cell gives rise in the rabbit to the inner mass of cells (si"!^ blow), which he terms the entoderm, but which must, it seems to me, be homolc^ized with the ectoderm, as explained helow. That Van Beneden is in error as to the genetic relation of the small ci'll to the inner mass has been demon8trate<l hy Heaiie, 86,1, Kill.

The second cleavage plane is probably also meridional, and is certainly at right angles to the first, so that four siniilar cells are pn>duced as in the primitive tj-pe of segmentation,* Fig. 54. These four colls are also rounded at first and probably liecome fittetl against one another so as to produce the diajxisition oh8er\-ed hy Tafani, 79.1, lin, in mice ova at this stage. Tafani descnbes each cell as having the form of a three-sided pyramid with the apex at the centre of the ovum and a convex base forming part of the external surface of the yolk. That the two first cleavage planes are meridional ia rendered probable by the arrangement in the four-cell stage observed by Selenka in the Virginian opossmn Fig 55

lliv* type i-t Demneutoliini " oml "|jriiiillivi. tv|Ki a mioJ ny llie reailer

■nty hours.

During all these earlv stages the colts (sf^mentetion spheres) are nake<l, i.e., without anj mem brane; the nuclei when not m karyokinetic stages are lai^ clear, and vesicular the }olk granules are small highly re fractile, and more or less nearl\ spherical ; they show a marked tendency to lie in the cell half way between thenucleus and the edge of the cell, or when the celb are large around the nu cleus and at a little distance from it.

It is at about this stage that the ovum passes from the Fal Ionian tube into the uterus where it dilates into what is known as the blasfodej ttnc vesicle. This dilatation is due principally to the multiplication

and flattening out of tiie cells of the outer layer and, of course, involves the expansion and consequent thinning of the zona pellucida, compare Figs. 56 and 58. The inner mass meanwhile remains passively attached to one point on the circumference of the vesiclo, Fig. 66, *'. m. By this process the thin fissure between the inner mass and the outer layer becomes a considerable space, Fig. 59, s. c, the cavity of the blastoderm or segmentation cavity (blastococle) .

W. Hmiw.

The blastodermic %-esicle continues to expand, and in the rabbit and mole there is a corresponding enlargement of the tubular uterus at the point where the vesicle ia lodged. *' It is clearly impossible for the delicate-walled o%'um to expand in the form of a vesicle, and distend the uteriue walls by virtue of the growth of ita cells; it must be, therefore, concluded that it obtains some support. Thia support is rendered from within. The vesicle coutaiiis a transparent fluid, the nature of which I am only sufficiently conversant with to say that after treatment with iilcohol a white precipitate is present in the vesicle. It is equally evident that this fluid can only have been obtained from the uterus, and that it is present within the vesicle at a very considerably greater pressure, than in the ut«rus itself. Such a condition is * caused by means of the cells of the wall of the vesicle; they secrete the fluid within the vesicle, this function being performed against a pres.sure which is greater on their inner than on their outer side, exactly as the cells of the salivary glands are known to act. The uterine fluid is secrete*! by glands present in great numbers in the uterine tissue, and is jraurcd through their open mouths into the '. cavity- of the uterus. There is every probability it has nutritive qualities, since it is thence taken up into the cavity of the embryonic vesicle, which eventually functions us a yolk-sac, in the walls of which embryonic blood-ves-sels ramify " (Heape).

The inner mass. Pig. 5ii, ;. «/., does not at first grow much and retains its rounded form, becoming, at least in the mole, nearly globular. Fig. 57, A. The inner mass subsequently flattens out, becoming lens-shaped, thinner, and of larger area, Fig. 57, B. It continues spreading laterally and separates into three distinct layers. The oiiim now consists of a very thin zona pellucida. Fig. 5S. 2, close against which is a single layer of thin epithelial cells, E}i: at one pole this layer is interrupted by a lens-shajied mass, i. m., formed by three layers of colls. These three layers were first clearly described bv E. van Beuedcn, 76. 1 , and have been since figured by him, 80. 1 ; Van Beneden identified these three layers with the three permanent gcnnlayers which do not arise until later. ItiHiber, however, «howe<l that both the outer layers enter into the formation of thi? ccto<lerm, while the inner layer is concerned in the production of the permanent entoderm; the outermost layer Haulier tenns the Dcck.schirh I. Lieberkuhn, 79, 1 , and others have since then confirmed Baul>er'8 results.

Homoliiijies of the ilaiuiiiuliun Blasfodermic Ve-ticle. — "We bave so little acourate iiifoiTnatimi concerning the details of the funnatioii of the bUistoclennio veaicle that any interpretation must be tentative. I still consider, however, the view which I brought forward in 18S5, " Hdbk," I., 5"Jlri, an the most satisfactory, and preferable to the similar explanation advanced independently and simultaneously by Haddon, 86.1, and repnxluced by him briefly in hie "Practical Embrj-ologj," 47, 48. F. Keibel, 87.1, advocated similar interpretations two years later, but without quoting Minot or Haddou. 1 regard the subzonal epithelium as the ento<krm and the inner mass of cells as the primitive bliistoderm or ectoderm; by so doing the parts can be readily and exactly homologized with the parts in the frog's ovum, as will be evident at once if the dia^am, Fig, 51t, of the mammalian vesicle be compared with the section of a segmented amphibian ovum. Fig. 40., The primitive blastoderm Bl, or ectoderm, consists of several layers of cells rich in protoplasm ; below it in the large segmentation cavity, a. c, relatively much larger in the mammalian than in the amphibian ovum. At its edge the primitive blastoderm ijoins the entodenn Fo/A-, which in amphibia is i^ large ma.'^s, in mammals only a single layer (if cells. Jfow, we know that the anct-stoi'S of the higher mammalia had ova with a large amount of deutoplasm, which in the course of evo hit ion has been lost, so that in th. ova of the placeii talia there is ver" little yolk-material: we know further that Ihireadiiiei-s of cellu lar divisions dejK'nds (.n tinamount of yolk, hence, when the yolk is lost, we should expect to find the entoderm, which, as we have seen, is vno JViihIch >. suli nnl inli. Luiih i m-l mi r unnpeilml

derived from the vegetative substanre of the o\um, to be represented b^ relatively small cells, if we im.igme the numbei of entoJermic cells in the frog's 0%-um. Fiti- 4'i I oik reduce*! their connection with the prim itive blastoderm and thnr character as . contmuou', la\er being preserv'ed, *ve obtani at once the chdracteri'-tic arrangement of the mammalian blastodermn. vesicle, Fig "j'I The homologj here established is further confirmed by the coarse network of protoplasm in the cells of the outer layer of the vesicle (Ed. van Beneden, 80. 1), suggesting at once the meshes which have been emptied of their deutoplasm. Adam Sedgwick, 86.1, has shown that in the ova of Peripatus capensis the yolk-matt^r has been lost, though abundant in other species of the same genus, and the coarseness of the protoplasmic network is preserved as evidence of the granules formerly present. This observation serves to confirm the view I have suggested as to the significance of the wide-meshed reticulum of the cells of the mammalian subzonal layer. Fig. 5'J, Yolk.

The disposition of the animal pole in the ovum before segmentation also conforms to the homologies here advocated. It will be remembered, ante^ p. 55, that the protoplasm of the animal pole extends far into the ovum and is enveloped by a cup (deutoi)lasm zone) of the substance of the vegetable pole. Hence, when the animal pole forms cells, they lie as an inner mass. Fig. 56, i.m. If Minot's view be adopted, then the ectoderm lies within the entoderm at a certain stage of development, for the one cell which retains, as shown in Fig. 53, the connection of the ectoderm with the exterior is subsequently overgrown by the outer layer of cells (Van Beneden, Heape). There is, then, a complete inversion of the germlayers in all (?) placental mammalia. In most cases the inversion is temporary-; the inner mass as described above flattens out, and probably flattens out inside the outer epithelial layer ; if this is the case then the external layer of the lens-shaped mass. Fig. 57, B and C, is really entoderm; this layer is Rauber's Deckschicht, which, as already stated, usually disappears, leaving the true inner mass or permanent ectoderm to form part of the surface of the blastodermic vesicle, so that with the exception of the reduction in the dimension of the entoderm the relations are the same as in other vertebrate ova. The inner layer of the flattened inner mass gives rise to the entoderm, and this at first sight appears to be conclusive evidence against the homology here drawn between the inner mass and the primitive ectoderm of other vertebrates. The same thing was formerly supposed to occur in the blastoderm of other vertebrates, but it is now known that the entoderm is added from another source to the under side of the primitive blastoderm or ectoderm, and though we possess no exact information whatever as to the origin of the entoidermic cells under the primitive blastoderm of the mammalia, there is no reason to assume that they arise in a manner fundamentally different from that typical of other vertebrates. We may therefore, dismiss this objection. The origin of the entodermic cavity and its lining is described in the next chapter.

Fig. SO.— Dia^^ram of a Begmented mammalian ovum. Z, zona pelluoida: ttl. primitive blaKtoderm; ».c., segmentation cavity: Yolk^ layers of cell representing the remnant of segmented yolk.

Planes of Division During Segmentation

The plane of the first division determines those of the subsequent divisions, and also perhaps the axes of the embryo;* it is itself determined by the position of the long axis of the first amphiaster or nuclear spindle to which it is at right angles. It, therefore, is a matter of great interest to ascertain what factoi's determine the position of the first spindle, or, in other words, the axis of elongation of the segmentation nucleus. So far as at present known, there are two factors: 1, relation to the axis of the ovum; 2d, position of the path taken by male pronucleus to approach the female pronucleus. The axis of the ovum is fixed before impregnation ; it passes through the centre of the animal and that of the vegetable T)ole. Usually the nuclear spindle which leads to the formation of the polar globule has its long axis coincident with that of the ovum, hence the point of exit of the polar globule marks one end of the ovetic axis. the first amphia^ter or spindle is always at right angles to the ovic axis. This, however, leaves the meridian plane undetermined. Roux, 87.1, from a series of interesting experiments on frogs' ova, concludes that the plane is fixed by the path of the spermatozoon. So far as I know this idea was first suggested by Selenka in 1878, in his paper on "The Development of Toxopneusters Variegatus ;■ ' compare, also, Mark, 81.1, p. 500. In the frog's egg the path of the male pronucleus is marked by a line of pigment, as was first described by Van Baml>ecke, 70. 1, 05, and has been well figured by O. Hertwig, 77.2, PI. v.. Fig. 48. The pigment renders it easy to ascertain the position of the male road even after the first cleavage of the oviun. This Roux has done in sectioned ova, and from experiments and observations reaches this result: The long axis of the first segmentation spindle lies in a jylane^ which jicisses through the axis of the ovum and the path of the male pronucleus. If Roux's conclusion is confirmed, it will Ijecome of fundamental imix)rtance. Yet there must be other factors which can at least replace the male pronucleus in this special role, since the development of parthenogenetic ova, in which there is no male pronucleus at all, is equally determinate. It is probable that the distribution of the protoplasm is the real cause determining the position of the nucleus; thus in oval eggs the spindle lies in the direction of the long axis ; it is quite probable that if the male pronucleus has the effect ascribed to it by Roux, it produces it indirectly by altering the distribution of the protoplasm within the ovum ; that such alteration takes place is indicated by the occurrence of the male aster.

That the first cleavage plane is determined by relations existing in the unimpregnated ovum, has been suggested by O. Schultze in consequence of his finding the germinal vesicle lying eccentrically in the eggs of the brown frog. Schultze suggests that the first plane passes through the ovic axis and the eccentric nucleus. Roux (Biol,

  • In certain owes, notahly m birds eh described alwve, the sefrmentation is irreinilar; and it la therefore not known yet whether the sclieme of arraiiKenieut of the oleavaee planes hiiere given can be applie<l to all* ova or not. We may say. however, that the scheme 18 tne primitive one. from whlclianv ino4lifications uroso phyloj^enetically. The »M»st discussion is by A. Agassiz and Whitman, W.l.'*»-41

Cbl., VII., 420), maintainB that this suf^estion is set aside by his own observations cited above. For further discussion see Schultze'8 sbortnote, 87-3, and Roux's rejoinder, 88.1. I think the question whether the first cleavage plane is determined by the ovum's structure or not is still an open one.

As already stated in the primitive segmentation, both invertebrate and vertebrate, the second cleavage plane is at right angles to the first and also meridional, while the uiird plane in at right angle^t to both tlio first and therefore equatorial. In meroblastic vertebrate ova thin regularity is entirely lost.

Relation of the Segmentation Planes to the Embryonic Axis. — It has been assumed by some writers that the first cleavage plane coincided with the future median plane of the embrj-o. This conception is rendered extremely improb<ible by the fact that the segments of the ovum have been observed to migrate in various cases so as to destroy the symmetrical grouping. Miss Clapp's observations, 91.1, i'.>'.>, OQ the toad-fish show that the me<lian plane of the embr3'o may form almost anj- angle with the first cleavage plane.

Differentiation of the Ectoderm and Entoderm.— As already pointed out, the essential feature of segmentation is the unlikeness of the cells produced; the manifold variations in the process of segmentation depend chieflv on the amount

f X Mmot in IST'T, 17, first estahlished the generalization that in all (iiiiinalt

f , till ormn undergoes n total scf/nienta an dnrin;/ irliich the cells <if ili<i erto

(lei in divide, faster and beaiine snniller than the cells of the entoderm: compare Fig. till. There are, however, a small, imd I think diminishing, number of cases, where the process uf segmentation IS imperfectly understood, and which oraiua™d^rDK"siiini'iiiai'i nsta^ cauiiot yet bo shown to Conform to this B Hn^*hek oLoc^S-pM generalization. "All the known variaby Urjco ent «i onin tha oilier by tions in the process of segmentation de sinall r eotoU r ual cells

pend merely uixtn : 1st, the tiegree of iliffertnee m size between the two sets of cells; ;id, the time when the difference appears; :fd, the mode of development, whether polar or by dclamination,* either of which may or may not be accompanietl by axiid infolding. In Ga8teroix)d3, Flanarians, *" CalcisjKjngije, Gephyrea, Annelida, fish, birds, and Arthropods, the difference is great and appears early. In Ecliinoderms, most Ccelenterates, some sponges, in Xematotls, Amphibians, etc., it is less marked and appears later."

In most cases the entotlermic cells are verj- decidedly larger and less numerous than thiwe of the ectoderm. This distinction is obviously necessarj- on account of the mutual relations of the two primitive layers. The ectoderm has to grow around the entoderm, which it can do only by ac^piiring a greater superficial extension; this the ectoderm accomplishes by dividing very quickly at first into small cells. After the ent<xlerm is fully enveloped it may then continue to grow until its superficies is much greater than that of the outer layer, within which, however, it still finds room by forming numerous folds; thus is gradually reached the condition in the higher adult animals where the intestine sometimes has an enormous surface, but is nevertheless contained in body- walls covered by ectoderm presenting much less surface. It is, therefore, only during the early stiiges of segmentation that we find the entoderm expanding more slowly than the ectoderm.

The tenns holoblastic and meroblastt'c are applied to ova according to their manner of segmentation. The first is employed for those ova in which there is either very little or only a moderate amount of yolk, so that the whole of the ovum splits up into distinct masses (cells) which enter into the comix)sition of the embryo. The second designates ova with a very large amount of yolk, so that while the protoplasm, from which the ectodenn arises, divides rapidly into distinct cells, the entodermal portion merely develops nuclei at first, with the result that while one j^ortion of the egg is " segmenting another portion (the entodermal) remains unsegmented, so far as the external appearances are concerned. Eggs, then, with much yolk, undergo the so-called partial segmentation; hence the adjective vierobla.stic.

Whatever the exact mode of segmentation there results always the same type of orgtmization, to which Minot has applied the term diaderni; it is characterized by consisting of two plates of cells, differing in character, joined at their edge (ectental line), and surrounding a central segmentation cavity ; the two plates or lamina are the two primitive germ-layers, the ectoderm and entoderm. The earliest form of the diadorm is that known as the blastnia, as Haeckel has felicitously named the first larval form of the lower animals. In the blastula we have a simple epithelial vesicle, the cavity of which is the large segmentation cavity, Fig. 47; the epithelial layer is one cell thick and divided into two regions; one ci>mposed of smaller cells is the ectixlerm, £c, and the other of larger cells is the entodenn. This stage occurs with sundry modifications in a great many invertebrates. These modifications are due principfiUy to the increiise in size of the entodermic cells, which, iis already pointed out, results from the increase of the yi|lk-matter in the ovum. Thus in many mollusks the entcxlermic cells are very large and at first few in number. Bv a still further modification the cellular yolk is replaceil by a mass rich in deutoplasm, but not divided into cells, while at the same time the segmentation cavity is re<luced by the invasion of the yolk-mass. In vertebrates we have the ailditional modification that the cells are several layers deep in the ectoderm and primitively in the entoderm also; compare the section of the axolotl's ovum. Fig. 41'; in certain forms, as we have seen, the entoderm is not divided into discrete cells, but remains one mass; this is the case in Elasmobranchs and the amniota, but in the highest amniota (Placentalia) the yolk is lost and the entoderm is again represented by a single layer of cells, Fig. 59.

It seems to me evident that the first step of development in the segmenting oinnn is the differentiation of the two germ-layers^ ectoderm and entoderm, resulting in the diaderm stage. Diaderm is a term preferable to blastula, because the latter is applicable strictly only to a special larval form, while the former is a general term which refers to the essential diflEerentiation at this stage. It is important to remark that the two layers are distinct in the diaderm or blastula stage ; it is often erroneously affirmed that the blastula consists of a uniform layer of cells, part of which subsequently becomes the entoderm.

The segmentation cavity comprises the whole space between the entoderm and ectoderm; it is very early invaded by cells produced from the two primitive germ-layers. These cells are in vertebrates of many kinds and enter the segmentation cavity at various periods. It is customary to group the cells which enter early into this cavity under the coipmon name of mesoderm, and to consider them as a third and distinct germ-layer. For convenience we may adopt this custom, for to a certain extent the mesoderm of authors is a separate germ-layer, but it by no means includes all the tissues which occupy the space between the two primitive germ-layers. As the space between the entoderm and ectoderm is always homologous with itself, it follows that the entire room between the epithelium (entoderm) of the digestive tract and its appendages on the one side and the epidermis on the other is homologous with the segmentation cavity.

The mesoderm of authors comprises three tissues: 1, free wandering cells {mesamceboids) ; 2, embryonic connective tissue or cells connected together by processes (mesenchyma) ; 3, epithelium, which forms two or more separate sacs. The origin of the mesoderm and the relations of the three tissues it contains are discussed in the next chapter.

The Gastrula Theory

In invertebrates with holoblastic ova the blastula passes into a stage known as the gastrula. Gastrula is, properly speaking, a new name for a larval form called planula by older writers ; but the term is now generally employed to designate an ideal embryonic stage, supposed to be common to all multicellular animals.

The blastula changes into a gastrula by a process of invagination. The entodermal area of the blastula flattens out, the ectoderm meanwhile expanding by multiplication of its cells ; after flattening, the entoderm turns inward, forming at first a shallow cup, then a pit which has an opening or mouth, the rim of which is the ectental line. The larva is now a double sac, and has an external wall or ectoderm and an internal wall or entoderm ; the entodermic cavity is entirely distinct from the segmentation cavity. The process of gastrulation is here described as it occurs among the lower invertebrates.

Typical gastrulse are the free-swimming larvae of many marine invertebrates ; we may take as an example that of a sea-urchin, Fig. 61. The larva is round; at one pole it has an opening, //i, the gastrula mouth leading into an internal cavity ; as this is a freeswimming larva it is provided with long cilia for organs of locomotion ; the cilia in many gastrulas are distributed over limited areas or they maj' be wanting altogether. The larva consists of a double SBC, a larger outer one of small epithelial cells, ec, the ectoderm, and a much smiUIer inner sac composed of larger entodermal epithelial cells, en; at the mouth, III, of the inner sac the two layers are continuous with one another; in the space between the two sacs, whicn corresponds to the segmentation cavity, are a few scattered cells, the first members of the mesoderm, tues. The entodermal sac of the gastrula is known as the archeiiteron ; other terms are also in use, e. g., mid-gut, coelenteron, urdarm, etc. The opening is known as the gastruta mottfh (arcbistome, urmund, etc.)The ctelenterates preserve tbe f,„. 8,._b*«J „t apu,t,i»«of To.

gastrula oreanization through- liTldi»; atter Selpnlia. «, actodBr- ■

out life, but in all higher classes "*""■ ■""• '"«»^'-'>'- "• '^'*'the archenteron gives rise not only to the permanent digestive tract, but also to many appendages and derivatives thereof ; and, moreover, the gastrula mouth closes o\ er, and in vertebrates the true mouth is an entirely new fonnation, which arises without any coimection whatever, so fsir as known, with the gastrula mouth. By gastrula^ tion the ectental line becomes the rim of the gastrula mouth.

A line passing tbruugh the centre of the mouth and the opposite pole of the gastrula is the so-called axis. Now, if the mouth be elongated, there would at once be a new longitudinal axis marked out, and the gastruLi would become bilaterally symmetrical. If, further, the mouth is pulled out into a slit, and in the process of evolution the lips come together and unite in their middle part, the animal would still have the two ends of the original mouth left open, and would so acquire two apertures to its archenteron — one anterior to ser\'e as mouth, and one posterior to serve as anus. This hypothesis of the conversion of a gastrula into a bilaterally symmetrical animal by the elongation of the mouth and concrescence of the lips or ectental line, was first suggested, so far as I am aware, by Rabl, 76. 1 . A very perfect exemplification of the process is afforded by the developing ova of Peripatus capensis, as shown by Balfour, 83 . 1 , and Sedgwick, 86.2, PI. XXSII., Figs. 23-26. There are, however, serious difficulties in applying the theory to bilateral invertebrates; I am strongly inclined to think that further research will obviate these difficulties.

In certain vertebrates and annelids the concrescence of the ectental line has been clearly demonstrated, hut the process is rendered by secondary modifications much more complex than that described in the preceding paragraph — the detailed account of it forms the subject of the next chapter.

TTie gastrula theory in that all metazoa have a common inherited stage of development, which follows immediately after the diaderm; this st£^ is characterized by there being an outer ectodermal sac with a perforation to the edge of which is attached the entoderm, which forms a closed inner sac, the archenteron. The embryolog}' of coelenterates teaches us that the gastrula is a secondary type, and thus the interesting problem of the origin of the gastrula is to be solved by the invertebrate embryologist (see J. P. McMurrich, 91.1,310.)

The term gastrula was introduced by Haeckel, and is now universally used by embryologists. The discovery of the importance of the ^^trula is due to the brilliant researches of Kowalewski on various invertebrates, including Amphioxus, then supposed to be a vertebrate. Haeckel then seized upon the idea of the gastrula and wrote an essay, 74.2, (compare also 76. 1), upon it, which from its brilliant style attracted notice, and did much to direct attention to the important disco verv of Kowalewski. Although Haeckel indulged his fantasy unduly and was misled into speculations which are now unheeded and almost forgotten, he did great good by starting the interest of zoologists in the right direction. By a remarkable coincidence, Lankester published an essay, 73.1, of very similar purport to Haeckel's, at about the same time.

The gastrula, like the diaderm, varies greatly, the chief modifications depending on the amount of yolk present ; this is illustrated b}^

the accompany ing diagrams, Fig. 62 ; the mesodemi is intentionally omitted ; A corresponds to such a larva as Fig. 01; the difference in size between the two sets of cells is slight but evident. In B, the difference is more marked, and fairly represents a gastrula of Amphioxus. In C, the diflEerence is very great and corresponds to that observed in certain gasteropod larvfe. In D, the inner set is no longer separated into distinct cells, although there are a number of nuclei, each of which marks the centre of a future cell ; in such instances we must regard the whole inner jx^rtion as not yet transfonned into a definite entodermic ceW-layer. This figure is particularly instructive, because it shows that what we call the yolk is not something distinct from the germ, but really belongs to the inner layer of the embr^^o. E sliows a similar egg, in which the outer set of cells has not yet grown around the yolk. F shows the same egg not in section, but seen from the outer surface in onler to exhibit the cap of small cells (blastoiienn) resting upon the yolk.

Fia. 04. — DiaKniuiH of the Princii>al McidiflcatiouH of thv Uantrula ctue text). A— E. repreaenU Hectioos.

This chapter was published in a preliminary form in the American Naturalist, June-August, 1890. Since then the researches of Van Beneden, 88.3, on flje rabbit, and of L. WiU, „ 89.1, 90.1, 92.1, on reptiles have cleared up n|many obscure points. The chief gain, as Prenant J| has shown in his ■' Embryolt^e, " is the knowledge that probably in all, certainly in many vertebrates (excluding Amphioxus) , the entodermal canal arises by the fusion of two cavities ; one of these is the long-known notochordal or blastoporic canal, which communicates with the exterior by au opening (blastopore) at its posterior end; the other cavity is formed in the yolk immediately underneath the notochordal canal and i.s completely closed. Verj' early the partition between the two cavities disappearr and they fuse, making the definite cavity of the enbxlermal canal. This primitive canal, from which the pharynx, lungs, and digestive oi^ans are dilTerentiated, is known as the , a rclienteron. The relations with which we are now concerned are illustrated by Fig. Ca.

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Cite this page: Hill, M.A. (2024, February 23) Embryology 1897 Human Embryology 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/1897_Human_Embryology_4

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