A textbook of general embryology (1913) 8

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Kellicott WE. A Textbook of General Embryology (1913) Henry Holt and Co., New York.

Kellicott (1913): 1 Ontogeny | 2 The cell and cell division | 3 The germ cells and their formation | 4 Maturation | 5 Fertilization | 6 Cleavage | 7 The germ cells and the processes of differentiation, heredity, and sex determination | 8 The blastxtla, gastrula, and germ layers. Morphogenetic processes

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Chapter VIII. The Blastula, Gastrula, and Germ Layers Morphogenetic Processes

In this chapter we shall endeavor to summarize the more general processes of early development which lead to the formation, out of the group of cleavage cells, of an embryo possessing the beginnings of the chief organs or systems. This will carry us from the formation of the blastula, through the important events of gastrulation and germ layer formation, and the varied processes by which tissue and organ rudiments are set apart and diflFerentiated.

We shall give particular attention, indeed practically shall limit ourselves, to a descriptive morphological account of these events. This is done, not as a matter of choice, but because the experimental results of the functional analysis of these processes are so fragmentary and so scattered, that the attempt at their summary, in a text of this character, seems unwise. This is partly because the efforts to analyze these processes experimentally have been delayed until the similar problems of cell organization and cleavage should have been more satisfactorily solved. These topics we have already considered. We should say, however, that what has already been accomplished in the way of describing these later phenomena of the mechanics of development {Entwicklungsmechanik, Roux) bears out the general conclusions indicated by the earlier processes, namely, that while the action of external conditions as stimuli is essential, their place normally is chiefly that of affording the general conditions of life and development. The actual quality and the really significant details of the later, as of the earlier phenomena of development and differentiation, depend primarily upon internal conditions and relations.

And further, there is little reason for supposing that here too the essential determinative conditions are other than nuclear, although from the nature of the case, the evidence must be less direct.

We shall limit ourselves in another direction also. It is obviously impossible to give a brief, and at the same time an adequate, account of the extremely diverse methods of gastrulation, germ layer formation, efc., in the Metazoa as a whole. We shall, therefore, confine ourselves here largely to the consideration of these events among the Chordata. This will enable us to give a more adequate consideration to the topics selected.

We have seen that while no definite termination can be placed to the period of cleavage, there is rather general, though arbitrary, agreement that cleavage may be said to have terminated when the blastomeres become arranged as a more or less definite epithelium or layer, bounding an internal space of some sort; and further that this may, in some cases, also be marked by the attainment of a certain nuclear-cytoplasmic relation. The organism exhibiting these characteristics is known as the hlastvla. In this stage the organism is essentially a monodermic structure, that is, it is composed of a single tissue or layer of somewhat similar cells. In the simplest form of blastula this layer is but one cell thick (Amphioxus, Fig. 150, A), but in most of the Chordate blastulas the wall is many cells in thickness.

Like the cleavage pattern, though to a much greater extent, the general form of the blastula is largely determined by the amount of yolk or deutoplasm contained in the ovum; and the form of the blastula, in turn, largely determines the form of the gastrula, and the methods of gastrulation and germ layer formation. We may for convenience, therefore, distinguish three general forms of blastulas, although intergradations are not infrequent. When the ova are nearly homolecithal, and cleavage is total and adequal, as in Amphioxus (Fig. 160, A), the blastula is practically a hollow sphere (cceloblastula). Its wall is a simple epithelium, one cell in thickness, and its cavity, the blastocael or segmentation cavity, is large and nearly central, though not quite, for nearly always the cells at the vegetal pole are larger than those at the animal pole, and the wall consequently thicker in the former region. This form of blastula ia commonly regarded as primitive, though hardly typical of the Chordates in general, for among these it is found only in Amphioxus.

Fig. 150. — Tyi)ea of Chordate blaatulte. A. Amphioius (cceloblastula).. B. Pettomyzon. After von Kupfter. C. Notume (Teleost) (disco blastula). D. Triton (Urodele). Afl«r Greil. a, snimal pole; c, blaatoccel; p, periblast; V, vegetal pole.

More usually the Chordate ovum contains a considerable amount of yolk, as in the Ganoids and Amphibia. And here the cleavage, though nearly or quite complete in most cases, is decidedly unequal and the form of the blastula is considerably modified in consequence. Here the wall of the blastula ia several or many cells thick; the cells of the animal pole are quite small, while the yolk-containing cells, of the vegetal hemisphere, are very large. The blastoecel is therefore quite eccentric, displaced toward the animal pole, and usually much reduced in size (Fig. 150, B, D).

The most highly modified type of Chordate blastula is found in those forms with extremely meroblastic, telolecithal eggs, where cleavage is of the discoid type. This condition is common to the Elasmobranchs (Fig. 158), the true Teleosts (Fig. 150, C), and to the higher Craniates — the Reptiles, Birds. In reality this is, in a modified way, characteristic of the Mammals also, for although the Mammalian ovum is nearly alecithal, it is clearly derived from the Reptilian condition, and many features of its development show unmistakably the effects of a large yolk content previously present, but now lost in correlation with the newly acquired source of nourishment possessed by the Mammalian embryo. The result of discoid cleavage is the formation of a small mass of living active cells, the blastoderm, or blastodisCy or germ disc, lying upon the surface of the yolk mass. The blastula instead of being spherical, has therefore the form of a circular disc, the cellular elements of which can really be compared, at first, only with the cells of the animal pole of the spherical blastula, the unsegmented yolk representing, in this stage, the large cells of the vegetal pole of such a blastula. In comparing these two types of blastulas we may imagine that the ordinary spherical blastula has been cut in two horizontally, through or just above its equator, and the animal hemisphere flattened out, its circumference being thereby somewhat extended. This form of blastula {discoblastula) is several cells in thickness and is usually separated from the underlying yolk by a shallow space called the sub-germinal cavity which represents the blastoccel (Fig. 150, C). While the yolk mass is usually by no means wholly devoid of nuclei, these may or may not be associated with true cellular structures, and even when present at this stage these rarely give rise to any structural parts of the definitive embryo when this forms. The Mammalian blastula diverges widely from any of these conditions and on account of its very special character further reference to it may be omitted here.

The next important step in development consists essentially in the conversion of this monodermic blastula into a didermic organism, that is, one in which the cells are arranged in two, more or less distinct, tissues or layers. This process is knowa as gcLstridation, and the didermic embryo itself is called a gdstrula.

Among the lower forms the process of gastrulation often remains a simple one, involving little more than the mere rearrangement of the cells of the blastula into two nearly homogeneous layers. But in the higher forms, such as the Chordata, with which we are dealing, the process is greatly complicated by the precocious formation of the rudiments of the chief axial structures of the later developing embryo, as well as by the dififerentiation of a third tissue, or intermediate layer between the other two. The establishment of the rudiments of the axial notochord and central nervous system, characteristic structures of the Chordate embryo, is termed notogenesis. These rudiments are formed out of the substance of the two primary layers of the gastrula. But the formation of a tissue between these converts the didermic embryo into a tridermic organism.

It is possible to analyze the whole process of development during this period into these three subsidiary processes, gastrulation, notogenesis, and middle layer formation, and in some instances they may occur somewhat separately and successively. But usually there is much overlapping, and the attempt to describe the process of gastrulation by itself would give a very incomplete and incorrect view of the events of this period. Consequently we shall describe all three of these processes together.

Three general types of gastrulas and modes of gastrulation may be found, corresponding with the three types of blastulas and ova mentioned above. Again the simplest and probably the most primitive, though not the most typical, condition is found in Amphioxus. On the posterior side of the blastula, in the region just between animal and vegetal hemispheres, Cerfontaine has described a small group of cells marked by a tendency to rapid and continued multiplication (Fig. 151, A). This region of active proliferation gradually extends laterally around the blastula and ultimately forms a nearly complete ring, though this is not until the blastula has become converted into the gastrula. This specialized group of cells may be termed the germ ring, for it is evidently equivalent to the structure already known by that name in the Fishes, Amphibia, and other forms. At the time this rapid proliferation commences, the vegetal hemisphere becomes flattened. The large cells of this region then arch up slightly into the blastocoel and soon begin to fold, or swing, inward about their postero-ventral margin as a relatively fixed point (Fig. 151, B, C, D). This motion is made possible by the rapid extension of a sheet of cells which come off from the germ ring and which are thus drawn in, to line the inside of the animal hemisphere. Without going into details here, we may say that finally the intuming of the vegetal cells becomes complete (Fig. 151, E) and the blastula is converted into a cup-like structure, widely open toward one side (the posterior or postero-dorsal).

The wall of the organism is now composed of two layers or epithelia, the original blastocoelic cavity is nearly or quite obliterated, and a new cavity is formed, lined by the inturned cells, and widely open to the outside. This structure is the didermic gastrvla. The two cell-layers composing its wall are the primary germ layers. The newly established gastrular cavity is the archenteron or primitive gut cavity. The superficial layer of cells, including the original animal hemisphere of the blastula and also some cells derived from the proliferating area, is known as the outer germ layer, or ectoderm, or ectoblast, or epihla^t; the layei' lining the archenteron, partly the cells of thie vegetal hemisphere of the blastula and partly the cells derived from the proliferating region, is known as the inner germ layer, or endoderm, or entoblast, or hypoblast. The opening of the archenteron to the outside is the blastopore; the periphery of the blastopore is spoken of as its m^argin or lip, and we have seen that it is the region largely occupied by the germ ring: it is here that the two primary germ layers are directly corvtinuov^ with one another.

The process of infolding, such as is carried out here by the vegetal cells which come to line the ventral region of the archenteron, is known as invagination] and in some of the lower forms the endoderm is wholly formed by this process. The process of intuming, such as results m the Immg of the dorsal region of the archenteron by the cells derived from the margin of the blastopore, is known as involution. In some forms the endoderm is largely formed by this process. Usually this is accompanied by the growth of the margin of the blastopore, or germ ring, on over and past the involuted region, so that a layer of cells is continually being overgrown, leading to more extended gastrulation; this process of overgrowth may be termed ejnholy. The blastopore of the Amphioxus gastrula, at first widely open, soon closes rapidly on account of the growth and epiboly of its lip. This process leads also to the rapid elongation of the gastrula (Fig. 151, F-H).

Fig. 151. — Goatrulation in AmphioiuB. After Cerfontune. A. Blastul* ■howing flattening of the venetal pole and the rapid proliferation of ceils in the po8tet<v<]otaal region (germ ring). B, Flattening more pronounced; mitoeea in cells of germ ring. C. Commencement of the infolding (invagination) of the cells al the vegetal pole. D. Continued infolding and inflection, or involution, of ectoderm cells in the dorsal lip of the blastopore. The hlastociel becoming obliterated and the archenteron being established. E. Invagination complete. Continued involution in the dorsal lip of blastopore. Mitoses in germ ring. F. Constriction of blastopore and coniniencement of elongation of the gaatrula. Remnants of blaatoocel in ventral lip of blastopore. O. Gaatrulation completed. Continued elongation, and narrowing of blastopore. H. Neurentetic canal

Although we have included involution and epiboly as gastrulation processes they are here more properly to be regarded as processes leading to the formation of the notochord and the intermediate layer. For as the gastrula continues to elongate these structures begin to differentiate in the anterior part of the roof of the archenteron, from that part of the inner layer formed by involution and epiboly. Along the dorso-lateral regions of the archenteron appear a pair of folds out of the archenteron, each containing a narrow groove (Fig. 152, B, C), These folds are the rudiments of the intermediate layer, or mesoderm^ or mesoblast; and the grooves, known as the enterocodic groo))e8, are the rudiments of the ccelom, the cavity of the mesoderm. That portion of the archenteric roof between the mesoderm folds, later becomes folded outward and forms the rudiment of the notochord. The remainder of the archenteric wall, namely the ventral and ventro-lateral regions formed by the invaginated portion of the inner layer, forms the primitive gut or enteron. Whether we choose to call the involuted region really endoderm or not, is immaterial; if we do, then we must say that the chorda established by overgrowth of neural folds. Continued mitosis in germ ring. a, animal pole; ar, archenteron; 6, blastoporal opening; ch, rudiment of notochord; dl, dorsal lip of blastopore; ec, ectoderm; en, endoderm; gr, germ ring; nct neuenteric canal; n/, neural fold; np, neural plate; s, blastocoel or segmentation cavity; v, vegetal pole; vl, ventral lip of blastopore; //, second polar body.

Fig. 162. — Traosverge Bections through young embryoa of Amphioxua, ahowing formation o( nerve cord, notoehord, mid mesoderm. After Cerfontaine. A. CommeDcement of the growth of the superficial ectoderm (neural folds) above the oeuTBl plate (medullary plate). B, Continued growth of the ectoderm over the neural plate. DifFerentiatiou of the notoehord. and first indications of mesoderm and enterocoelic cavities. C Section through middle of larva with two somites. Neural plate folding into a tube. D. Section through first pair of mesodermal somites, now completely constricted off, E- Section through middle of larva with nine pairs of somites. Neural plate folded into a tube. Notoehord completely separated. Is the inner cells of the somites, muscle fibrillie ore forming (oompare Fig. 153). or, archenteron; e, enterocoel; eh, notoehord; ec. ectoderm; en, endoderm;/, muscle fibrillte; g, gut cavity; m, mesoderm (gastral); mi, mesodermal somite; nc. neural canal; nf, neural fold; np, neural plate (medullary plate) ; rU, neural tube.

and mesodenn are both derived from endoderm and the process of involution is to be regarded as a true gastmlation process.

But only the leaser part of the mesoderm is formed in the way just described. This part of the mesoderm is known as the gastral or paracfwrdal, or axicd mesoderm. If we trace the mesoderm folds, just described, posteriorly, we can follow them into the repon of the germ ring or blastopore lip which has now become considerably thickened on account of its contraction, and consists of a more or less undifferentiated mass of cells. This mass now passes almost entirely around the blastopore, laterally and toward the ventral side. The rapid proUferation of the cells of the germ ring has early led to the disappearance of the original simple epithe^, lial arrangement of its cells (Fig. 153), but as it moves backward, with the elongation of the gastrula, it leaves behind it (i.e., anteriorly from it) its cell products, which rapidly become differentiated into certain layers. On the

Fig. 153.— FronWl section ■' • i (4. through larva of AmphioxuB, suTiacc Of the embryo a layer is leit withBixpairflo/mesod^mai jj j^ ^ directlv coutmuous with the

Bomites, at the level 01 tbe ■'

notochord and somitea. ectoderm of the Original gastrula dearchenteron: e. enterooffit: rivcd from the animal hemisphere of n, notochord; g",8«, first and ^jjg blastula. Another layer is differ Bulh meaodermal Boraites.

entiated linmg the archenteron and continuous with the layer already there, and consisting of a ventral region continuous with the invaginated layer or true gut ■endoderm, a dorsal region continuous with the involuted layer forming the rudiment of the notochord, while dorso-laterally, between these two regions, the inner sheet is continuous with the mesodermal rudiments described above. In other words, out of the germ ring there are gradually differentiated, true covering ectoderm, gut endoderm, chorda] cells, and mesoderm. The mesoderm formed in this way, directly from the germ ring, is called peristomial mesoderm, to distinguish it from the gastral mesoderm formed in connection with the enterocoelic grooves. Very soon, as we have seen, the region which gives rise to the peristomial mesoderm comes to extend nearly to the ventral side of the blastopore region. The coelomic spaces of this peristomial mesoderm are not formed as derivations of the archenteron; they result from rearrangements of the mesodermal cells, and are entirely independent of the enterocoelic portions of the ccelom in their origin, although the two become continuous later (Fig. 153). These two forms of mesoderm are directly continuous with one another and have indeed a common primary origin, the germ ring or margin of the blastopore. If we recognize the essential difiference between them as that of time of formation, the altered circumstances surrounding the formation of each due to this time difference, become of secondary importance as regards our real conception of the mesoderm and its relations to the other germ layers. Thus the relation of both chorda and mesoderm proper to the cells of the monodermic blastula is the same as that of the endoderm proper.

Stated in a word then, the gastrulation of Amphioxus is a combination of invagination and involution, accompanied by epiboly, and the processes of notogenesis and mesoderm formation are intimately bound up with the formation of the inner layer.

Having become familiar now with the general ideas of gastrulation and the terminology of the process we may consider the remaining forms of this process in the Chordates much more briefly.

Our second type of gastrulation, as it occurs in the Amphibia and Ganoid Fishes, may be easily understood by comparison with the preceding. The chief differences result from the accumulation of yolk in the vegetal pole of the ovum and blastula, and the consequent comparative inertness of this region. That is, the chief modifications of the typical process of gastrulation appear in respect to the behavior of the lower pole, destined to form the inner layer of the gastrula.

In the Amphibian blastula, the form of which was described above, a region of more actively dividing cells can be distinguished extending around the fully formed blastula just above its equator, that is, between animal and vegetal poles (Fig. 154) ; this ia termed the germ ring, although it is frequently not a complete ring, being interrupted on the anterior side of the blastula. Gastnilation commences by the true invagination of cells just below the germ ring on the posterior side. The inertness of the large yolk-filled cells of the vegetal hemisphere greatly impedes the process of invagination and it is never carried very far. Involution occurs extensively in the dorsal region and is accompanied by very pronounced epiboly {Fig. 155); this latter process is very marked in the lateral and ventral regions where little or no invagination occurs. All of these processes are relatively less extensive than in the ccelogastnila of Amphioxus however, probably on account of the larger amount of yolk contained in all of the cells. Their place in development is taken, in a way, by a new process, namely, detamination. This is a process of splitting, whereby an extended mass or sheet of cells becomes cleaved apart by the rearrangement of the cells into two more or less distinct layers, separated by a definite space. This process of delamination begins where the processes of invagination and involution leave ofE, and it is important to recognize that the didermic character of the gastrula of the Amphibian results partly from all three of these processes. The chief result of involution is the formation of the rudiments of the notochord and gastral mesoderm, as in Amphioxus. Figure 155 shows how the blastocoel is finally obliterated by the invaginated and involuted regions. The germ ring finally completes its growth over the yolk cells or endodermal floor of the archenteron, and closes together much as in Amphioxus.

Fig. 154. — Section throuKh late blutuln of Irog, showing location of germ ring. Lat«r the germ ring is thickened and contracted, forcing the yolk cells upward into the segraentBtion cavity. After O. Schultse. a, animal pole; tn*. germ ting; p, pigment; <. blastocoei; v, vegetal pole.

Fig. 155. — Seriea of diagrammatio drawings of sections showing the proceaa of gastnilatioQ in the Urodele, Triton. After Greil and Buffini. F. TiaoBveim section, others sagittal. A. Blastula. B. Commencement of gBBtnitatJon (invagination). C. Continued invaginBtion accompanied by epiboly and involution. Formation of archenterou. D. Continuation of all three procesaea of gBstnjlation. BloatoctEl nearly obliterated. E. Aichenteron completely formed. Rudiments of notochord and neural plate differentiated. F. Transverse section through embryo shown in E, through the plane marked #. a, archenteron (gut cavity in F) lined with endoderm; 6, blastopore; ck, rudiment of notochord; «, ectoderm; en, endoderm; ff, plane of section shown in F; gm, gastrat (axial) mesoderm; n. neural plate (in F, bounded by neural folds); Tim, peristomial mesoderm; e, blastocisl or segmentation cavity; x, marks corresponding points on the surface ectoderm, showing extent of epiboly: v. yolk cella.

The formation of the mesoderm offers some points of difference when compared with Amphioxus. The peristomial mesoderm forms typically in the margin of the blastopore, out of the undifferentiated cell mass of the germ ring. Sometimes, in the region just within the blastopore dorsally, traces of enteroccelic outgrowths can be seen (Fig. 156), but most of the gastral

Fla. 156. — Part of a, section through the body of ao embryo of the frog, Rana futca, showinE traces of enteroccel formation. After O. Hertwig. a, arehen~ teron: c, enteroccels; ec, ectoderm; en, eododerm; m, mesoderm; n, notoebord; p, neural plate; y, yolk cells.

mesoderm is formed either from involuted cells derived from the germ ring, or later from the surface of the endoderm by a process of delamination or splitting off of the superficial cells lying next the ectoderm; these come off first as a solid sheet, which much later itself splits into two layers leaving a coelomic cavity between them.

Thus while invagination occurs to a slight extent, gastrulation in these forms results largely from the processes of involution and epiboly combined with delamination. The didermic condition results, to a considerable extent, from the overgrowth of the animal hemisphere cells (germ ring) which come to enclose the yolk cells of the vegetal hemisphere. As in Amphioxus, however, the yolk, although here so much more abundant, is finally included in the floor of the gut cavity, and the yolk cells take a direct share in the formation of the structures of the later embryo. After the mesoderm and chorda have been formed from the roof of the archenteron, this is left as a thin layer, only one cell in thickness, quite in contrast with the thick mass of cells forming its floor (Fig. 155).

Turning now to the third type of gastrula, that formed from the discoid blastula, we find conditions which vary widely from the Amphioxus type, but which after all may be interpreted in the light of the processes just outlined. In the Ganoid or Amphibian, both the animal and vegetal hemispheres of the egg share directly in the processes of cleavage, and blastula and gastrula formation; and the yolk, contained in typical cells, is carried directly into the wall of the primitive gut. But in the extremely meroblastic eggs of the Elasmobranchs, Teleosts, Reptiles, and Birds, the large yolk mass, which is the equivalent of the vegetal pole of the egg, does not cleave (Figs. 150, 48) and takes no direct share in the formation of the cellular blastula and gastrula. For comparative purposes, therefore, we have already seen that we must recognize the germ disc or "blastula" of this type as equivalent only to the animal hemisphere of such a form as the frog or Amphioxus, flattened out and resting upon the undivided yolk mass. In such a condition as this the equivalent of the germ ring would be found forming the periphery of the blastodisc. The process of gastrulation concerns only the blastodisc, for the yolk mass takes no more share in this than in the later processes of development; the gastrula forms separately from the yolk, which is left outside the embryo and as we shall see, comes into relation with it only indirectly. The blastnla or blastodisc, once formed as a flat plate, many cells in thickness, begins to extend over the sm*face of the yolk mass. Its central part becomes quite thin in consequence, but the margin of the disc, or germ ring, remains thickened, and as it advances over the yolk its margin becomes slightly involuted, forming a narrow shelf of cells on its inner surface, toward the yolk (Fig. 157). This inner layer is the rudiment of the primary inner layer. Gastrulation is thus primarily accomplished by involution. The margin of the germ disc is clearly the germ ring or rim of the blastopore, although its form and relation to the yolk are quite unlike what we have seen heretofore.

Fig. 157. — Sagittal section through early gastrula of the catfish, Ameiurua. en, endoderm; gr, germ ring; p, periblast; «, segmentation cavity, or sub-germinal cavity; y, yolk.

Fig. 158. — Semi-diagrammatic drawings of sections through Elasmobranch embryos. After Greil, after RUckert and Ziegler. A-D, Sagittal sections; E, F, transverse sections. A. Diagram of section to show relations of blastoderm to animal pole of blastula with less yolk. B, Commencement of gastrulation by invagination, epiboly, and involution., C, D. Continuation of all three processes of gastrulation. E, Transverse section showing relation of endoderm to yolk, mesoderm, and the germ ring. F, Transverse section farther forward, through stage resembling D, Anterior margin of blastoderm toward the left in A-D. o, archenteron; c, vestiges of enterocoels; cA, rudiment of notochord; ec, ectoderm; en, endoderm; qw,, gastral (axial) mesoderm; gr, germ ring; m, mesoderm; n, neural groove, bordered laterally by neural folds; pm, peristomial mesoderm; s, blastoccel or sub-germinal cavity; y, yolk.

Fig. 159. — Diagrammatic drawings of sagittal sections through embryos of Sauropsids. After Greil, after Will and Schauinsland. A, B, Two stages of Reptile. C Bird, a, archenteron; cA, rudiment of notochord; en, gut endoderm; pm^ peristomial mesoderm; «, blastocoel or sub-germinal cavity; y, yolk.

The Elasmobranchs and Reptiles afford important transitional conditions here, in that a definite process of invagination is indicated (Figs. 158, 159). Invagination is here limited to the posterior margm of the blastoderm, where the germ ring becomes elevated above the yolk as the endoderm is folded under the ectoderm. In the Teleosts little or no indication of invagination can be fomid. In these forms the germ ring extends rapidly over the yolk, reaches and passes an equator of the egg, and then as it continues to advance, gradually narrows, and finally closes completely, having passed over the entire yolk mass (Fig. 160). This overgrowth of the blastoderm occurs more rapidly in the anterior and lateral directions than in the posterior direction, so that the blastopore finally closes in nearly the same relative position as in the frog and in Amphioxus, i.e., postero-ventrally. As the germ ring extends around the yolk, only a single, and very thin, layer of cells is left behind it as a covering layer. In the posterior and posterolateral regions alone, is the involution of an inner layer well marked. It should be noted that in the Teleosts the endoderm is. largely replaced fimctionally by a specialized protoplasmic region on the surface of the yolk, known as the periblast, which contains free nuclei derived originally from those of the margin of the blastodisc (Figs. 150, C; 157).

During the later stages of the overgrowth of the germ ring, as it contracts after passing the equator of the egg, its substance is payed into its more slowly advancing posterior region, where it forms a longitudinal median thickening (Fig. 160). This thickened region of the blastoderm is the primitive streak, the earliest rudiment of the essential parts of the embryo, which gradually differentiate out of its anterior end.

In such a gastrula as this the endoderm forms a flat median plate of cells Ijnng directly upon the surface of the periblast (yolk), and the archenteron is present only virtually as a narrow space between the endoderm and periblast (Fig. 157). In such a case the formation of a true gut cavity is independent of the formation of the inner layer, and occurs later by a process of folding.

The mesoderm is differentiated at a comparatively early stage, and the distinction between peristomial and gastral mesoderm is very clear. The peristomial mesoderm appears as a small mass of slowly differentiating cells lying in the germ ring, between the superficial ectoderm of the blastoderm and the involuted shelf of endoderm (Fig. 158, F), The gastral mesoderm is seen budding off laterally from the primitive streak region, also between ectoderm and endoderm or even beyond the region where the endoderm is found (Fig. 158, F). Posteriorly of course the gastral mesoderm of each side becomes continuous with the peristomial mesoderm, and as peripheral portions of the germ ring, where the three layers are* slowly differentiating, are continually passing into the posterior end of the primitive streak, it is clear that peristomial mesoderm is constantly becoming gastral, merely through relative change of position, not through any change in the mode of its formation or in its relation to the other germ layers.

Fig. 160. — Diagrams of the formation of the Teleost embryo by confluence of the germ ring, and the growth of the germ ring around the yolk. From Kopsch. A, In half -profile. B, In profile.

The Elasmobranchs and Reptiles are again transitional in that vestiges of enteroccelic grooves may be seen as shallow and narrow longitudinal depressions, either side of the midline, in the region of which the formation of mesoderm is most rapid (Fig. 158, F). In the Teleost, as in the Bird, no traces of enterocoels are to be seen. As usual the notochord forms from the cell mass lying between the rudiments of the gastral mesoderm, and may be said to have been derived either from the gastral mesoderm, or from the endoderm in the same way that the mesoderm itself is. After the separation of the chorda and mesoderm, the endoderm proper, or enterodemij as it is called, is left as a thin narrow strip of cells spread flat over the periblast (yolk) surface, continuous posteriorly with the diverging limbs of the germ ring.

In the Sauropsids, where the accumulation of the yolk is most pronoimced, the blastoderm does not grow entirely around the yolk until long after the gastrula is formed and the embryo established. Correlatively we find no typical germ ring formation in the periphery of the blastoderm, save in that posterior region which is to be concerned in embryo formation. Remembering that in the Reptile both true invagination and enterocoel formation occur, while these processes are not apparent in the birds, we may describe (following Patterson's account) the processes of gastrulation and embryo formation in the pigeon, as illustrating these events in the development of the extremely meroblastic ovum.

The blastoderm first becomes quite thin, particularly toward its posterior side, where, at the same time, the margin thickens forming a segment of a true germ ring (Fig. 161). The extension of this posterior part of the germ ring, however, involves the usual processes of cell multiplication accompanied by involution and epiboly; there is no true invagination here (Fig. 161). The formation of an inner layer is thus limited to the posterior region of the blastoderm. Soon, as this whole region extends posteriorly, this segment of a germ ring begins to contract toward the mid-line, and the result is the formation of a median thickening in the posterior half or third of the germ disc. This thickening is the primitive streak (Fig. 161), and as usual it is the seat of the formation of the chief embryonic rudiments. As in the Teleost, the primitive streak, formed by the gradual fusion of the lateral halves of the germ ring, is obviously the equivalent of the blastoporal margin of the frog or of Amphioxus. On its surface is a shallow longitudinal groove marking the separation of the two halves; this is the primitive groove (Fig. 161), which may be regarded as representing the blastopore proper. The archenteron, in such a gastrula as this of the chick, may also be said to exist only virtually, for it is represented only by a shallow space left between the endoderm and the yolk.

The mesoderm and chorda are here more closely related with the outer than with the inner layer. In the germ ring there is little indication of separation of the genn layers, other than the distinction of the endoderm, and when the primitive streak is formed, it appears rather as a thickening of the ectoderm. The mesoderm begins to be differentiated along the sides of the primitive streak, and back as far as the region where this is being formed by the fusion of the limbs of the germ ring. Hence the mesoderm is more largely gastral, that is to Say, it does not become distinct, as a separate rudiment, until the establishment of the primitive streak has occurred. In the germ ring there is of course a region where ectoderm passes into endoderm, and where the cells may be said to belong to either layer or neither layer. This is the region where the primary "mesoderm" formis and apparently special conditions may determine with which of the primary layers it may seem to have the more intimate relation. Little is gained by attempting to define germ layers in the germ ring.

In the pigeon or chick the rudiment of the notochord appears in the deeper part of the primitive streak after its lateral parts are cut off as mesoderm. The endoderm has therefore the value of an enteroderm from the beginning, and has the form of a very thin flat sheet of cells widely spreading over the yolk surface. Ultimately the yolk mass becomes entirely enclosed in a layer of endoderm as well as by the other germ layers, but this does not occur until a comparatively late stage in the development of the embryo.

In Amphioxus and the frog we have seen that the embryo is formed from the entire ovum, that is, the yolk-containing cells become actually included within the wall of the gut. In the Teleosts the yolk mass is so large, and so completely separated from the embryogenic tissues, that the embryo may be said to develop upon the surface of the yolk, which, enclosed within a structure called the yolk sac, is only indirectly related to the embryo proper. In forms like the Elasmobranchs and Sauropsids, the accumulation of yolk is still greater and the embryo forms quite apart from the yolk, with which it later acquires a secondary relation. In the Sauropsids, after the rudiments of the embryo are well established, a process of folding begins and a series of infoldings of the ceUular blastoderm, anterior, posterior, and lateral, pinch oflf the embryo from the yolk mass or yolk sac, with which it then remains only indirectly connected by a narrow tube known as the yolk stalk which includes a portion of the gut wall and a very abundant blood supply.

In the Sauropsids and Mammals other folds' of the blastoderm soon appear, beyond the limits of the embryo proper, which result in the formation of a very special and highly characteristic structure known as the amnion. And from the wall of the hind-gut grows out another special and extrarembryonic structure,, the aUantois, The formation and function of these extra-embryonic structures,. together called the embryonic appendages, cannot be described here. They are of the greatest importance in development and their presence has led to the application of the term Amniota to all the forms possessing them (Birds, Reptiles, Mammals) while the other Craniates, without these embryonic appendages (Cyclostomes, Fish, Amphibia) are tJien known as the Anamnia.

On account of the difficulties of comparison it seems wise to omit reference here to the Mammalian gastrula and germ layer formation. For in spite of the nearly alecithal condition of the Mammalian ovum, its development shows marked yolk influence, and the whole course of early development is complicated, not only through the one time presence and the subsequent loss of yolk, but through the very special relations of the early embryo, and particularly the embryonic appendages, with the walls of the maternal cavity in which development proceeds.


We should consider here, in a particular way, a developmental process which, besides being of great general importance in, Chordate development, is of considerable historical interest aa well. In the foregoing pages we have seen that where a germ disc is formed, its margm, known sb the germ ring, and recognized as the homolog of the lip or margin of the blastopore, is of great importance in the formation of the primary rudiments of the embryo.

The His- Whitman theory of concrescence emphasizes the general importance and significance of this relation. First stated fully by His, in 1876, the essential idea of this theory was that each side of the germ ring, not only forms, but really is the rudiment of the corresponding half of the embryo, which is thus actually formed by the approach and gradual, continued fusion posteriorly of the germ ring. In each half of the ring the essential rudiments of the embryo were thought to be already formed, partly at least, and the process of embryo formation consisted merely or chiefly in the junction or addition of these two originally separate halves. The anterior end of the embryo would thus be formed first, and embryo formation would be complete when the germ ring became fully contracted or closed. With some modifications of a really fundamental kind, this conception is widely adopted to-day. Both observation and experiment have shown, however, that definite halves of an embryo cannot be said to exist preformed in the lateral portions of the germ ring. These regions do contribute to the formation of the median thickening, known as the primitive streak or embryonic rudiment, by a process of gradual fusion posteriorly (Figs. 162, 163). But in this process of coming together, which may better be termed cojifivence (Sumner) than concrescence, the materials from the two sides of the germ ring are fused into a mass which is largely undifferentiated, and out of this the rudiments of the embryo appear, by a process of differentiation which occurs largely after confluence. One side of the germ ring contains not a half of the embryo, but the substance out of which, later, a half of an embryo forms. This process of differentiation is progressive and commences of course in that part of the primitive streak formed first, i.e., its morphological anterior end (Fig. 163). Then as the primitive streak lengthens posteriorly, the extent of the differentiated region at its anterior end similarly increases posteriorly, roughly keeping pace with the process of elongation (Fig. 164). The primitive streak thus may be regarded as a region which moves backward, receiving posteriorly the diverging limbs of the germ ring, and leaving anteriorly the differentiated rudiments of the embryo. Soon after the germ ring is completely closed or contracted, the primitive streak becomes wholly differentiated, and the rudiments of the embryo may be said to be fully marked out. The process of concrescence is seen most clearly and typically in those forms with large amounts of yolk and with well-marked germ ring, especially in the Teleosts and Elasmobranchs (Figs. 160, 165). In the Amphibia, where the amount of yolk is less, and the Sauropsida, where the germ ring is less marked, the process of concrescence, though somewhat modified and slightly obscured, still takes an important part in embryo formation.

Fig. 162. — Diagrams illustrating four stages in the formation of the Teleost embryo and the growth of the germ ring around the yolk mass. After O. Hertwig. e, embryo; gr, posterior margin of the germ ring; y» yolk mass; 1, 2, 3, 4» successive positions occupied by the germ ring as it advances over the yolk.

Fig. 163. — Diagrams of the formation of an embryo by confluence (** concrescence**). A, Germ ring before formation of the embryo is indicated. The letters a-e, represent symmetrical portions of the germ ring. B. Beginning of confluence. C. Embryo forming. AA, BB^ represent regions of the embryo, formed out of the materials of the germ ring at aa^ lib, D, E. Later stages in the formation of the embryo. The germ ring regions, cc^ and dd, have been differentiated into the embryonic regions,

Fig. 164, — Sai^ttol section through the hinder end of a flah embryo (Serrantia), BhowiDg the undifferentiated primitive streak, anterior to which the Htmcturea of the embryo are beiog differentiated. From E. V. Wilson, a.p. (v.l), anterior margin of blastoderm or ventral lip of blastopore, after having grown entirely around the yolk mass, bl., blaatopore; ec, ectoderm; en., endoderm; g.r,, germ ring; k.s., Kupffer'B vesicle; nc., notoehord; nr. cA.. nerve cord; p.. periblut; pp. (d.l.), posterior margin of blastoderm (dorsal lip of blastopore); pr. tlr., primitive streak.

Fig, 165. — BlastodertDB of the Elaamobranch, Torpedo, showinR formation of the embryo. After Ziegler. X 27. A. "Stage B." The poBtero-median thickening la the "embryonic shield," the first indication of the real embryo. B. "Stage C." Early embryo ; nerve oord rising above the surf ace of the blastoderm. In both figures the embryonic portion of the blastoderm is directly continuous postero-laterally, with the germ ring, which appears aa the thickened margin of the blastoderm.

The Germ Layers

While this is not the place to give an historical or critical account of the germ layer theory, it is important that the student should have in mind, before taking up the study of the development of particular organisms, certain fundamental conceptions of the germ layers, and their relation to development, particularly among the Chordata.

When the science of Embryology was itself in a very early stage of its development, the earliest differentiations recognized by the students of animal development, were the sheets or layers of tissue, such as those* of the chick, which seemed to give rise to the chief organs of the embryo. These layers of substance were described and their significance recognized, by such pioneers in embryology as C. F. Wolflf (1768), Pander (1817), and Von Baer (1828). The whole history of the formation of the systems and organs of the embryo could be read back to these layers, but beyond these, few constant structural features, susceptible of homolgy in diflferent forms, could be made out. The idea became very firmly fixed therefore, that these layers were really the primary diflferentiations of the embryo, and quite naturally their importance was strongly emphasized.

Subsequent to the statement and establishment of the cell theory, the genesis of the germ layers was traced, the blastula and gastrula fully described in a great variety of forms, and it was found that in spite of the greatest diversity in the earlier processes of development, the general character and structure of the germ layers remained remarkably imiform. And not only were the relations of the germ layers to one another quite constant, but their relation to the tissues and organs of the later embryo were subject to but little variation. In all forms inner and outer layers (endoderm and ectoderm) were present, and in all forms above the Ccelenterates a definite intermediate layer (mesoderm) was to be found. Moreover, in all of these forms the outer layer gave rise to the whole nervous system, central and peripheral, the essential parts of the sense organs, the epidermis and its appendages; from the inner layer came the lining of the digestive tract and its glandular appendages; while the intermediate layer gave rise to the sustentative, vascular, and muscular tissues throughout the body. All of this was finally developed, notably by the Brothers Hertwig (1879-1883), into a carefully and elaborately worked out Germ Layer Theory, the essential points of which were that the three germ layers are entirely homologous throughout the Metazoa, excepting only the Porifera (the Ccelenterates of course lacking a middle layer), and that these layers truly represent the primary and fundamental homologies in the structure of the

Metazoan phyla. Exceptions and contradictions were indeed occasionally noted, but their importance was minimized and they were treated frankly as exceptions, and put down to the account of "coenogenetic modifications of " palingenetic characteristics (see Chapter I) .

It is difficult to overestimate the influence of this theory upon the history of Embryology, and upon fundamental embryological ideas. Perhaps no conception, other than the general theory of evolution, has had greater influence in the field of descriptive embryology.

More recently, however, the limitations in the general applicability of this theory have been more fully recognized and the exceptions to the validity of its essential ideas emphasized. At present we must recognize the germ layers as representing a stage in development, just as do the blastula or gastrula, and of no greater or lesser importance than these. The germ layers are descriptive terms of the greatest importance, as such they are indispensable. They are not, however, starting points in any real sense; and to regard them as such is to look forward merely, not backward. Looking both forward and backward we see that the establishment of the germ layers is only one step in the continuous process of development. They represent no more essential homologies than many other features held in common by many developing organisms.

While we cannot consider in extenso the facts which have led to this change of opinion regarding the importance of the germ layers, we are boimd to state the nature of certain classes of these facts. In the first place are to be noted the difficulties of homologizing the layers of certain groups with their typical condition. For example, in the Porifera that layer which seems entitled to be termed the ectoderm, really gives rise to structures ordinarily derived from endoderm, while the "endoderm" itself forms the covering tissues. In the Mammals the "ectoderm may contribute little or nothing to the formation of the real embryo and the inner, outer, and middle layers cannot be exactly homologized with those of other Chordates, save by the grace of terminology. In the earlier part of this chapter the varied relations of the mesoderm to the other layers were mentioned; in some cases the middle and inner layers arise from a common rudiment, in others the middle and outer layers. Among the Invertebrates there are many instances of development where even the two primary layers are to be made out only with considerable difficulty, as for example, in the Trematodes, Cestodes, certain of the Bryozoa, etc.

In the second place the morphogenetic value of the individual layers is subject to a considerable variation. Thus in the Chordata, leaving aside the Mammals, the mesenchymal connective-tissue cells may be occasionally of "ectodermal or "endodermal," as well as of "mesodermal" origin. The endothelium of the heart may be "endodermal" or "mesodermal." The notochord may with equal correctness be described as endodermal, mesodermal, or even ectodermal, in various forms.

Single organs like the nephridia may be composites, ectodermal and mesodermal, or, in some cases ectodermal, in others mesodermal.

In the process of regeneration certain contradictions to the germ layer theory become apparent. Organs and tissues normally derived during embryonic development from a certain layer may, during regeneration, be produced from another layer. In certain Oligochaetes new mesoderm is of ectodermal origin, and the regenerated pharynx may be lined with endodermal, rather than ectodermal cells.

Especially in the process of budding, as it occurs in a great many groups, do we find abundant exceptions to this theory. In some of the Polyzoa the gut may be of ectodermal origin; the nervous system and pharynx are mesodermal in some of the flatworms. Analogous conditions are very common among the Tunicates; here the pharynx may be endodermal or ectodermal; the atrium and even the nervous system may be ectodermal, mesodermal, or endodermal, in different forms where in egg development the relations of these structures to the germ layers are typical.

Finally, the most important qualifications and limitations of the germ layer theory grow out of the observed facts of normal development prior to the formation of the germ layers. These structures are by no means the earliest constant embryonic differentiations, and as we have seen in the chapter on cleavage, it is just as easy to draw homologies between cell groups in the blastula stage, or in an earlier cleavage stage, as it is between the later appearing germ layers. It is not too much to say that in some cases homologies may be drawn between various formed substances in the undivided egg. Animal and vegetal poles of the ovum, cleavage patterns, cell-groups, micromeres, macromeres, upper and lower poles of the blastula, are all constant and comparable features of development no less than inner, outer and middle germ layers. We may recall that the cell known as 4d may be identified and its history and fate compared, in the cleavage of many groups, even in different phyla. This cell whose form, position, and derivation are so constant, may or may not form "mesoderm; even when it does form mesoderm this may go to form very different parts of the embryonic and adult structure. Often the "mesoderm" may be a cellj just as truly as a layer.

Summarizing we may say that while the arrangement of the cells of the embryo in the form of definite layers is almost universal, at the same time, in the comparison of different groups or of different modes of development, these layers exhibit great inconstancy in their relations to one another, and to the structures forming them and formed from them. The germ layers are valuable, indeed indispensable descriptive units, but they do not represent primary differentiations, and their homologies are no more, though probably no less, fundamental throughout groups larger than phyla, than are many other structures of the developing organism.

Morphogenetic Processes

It remains now to describe some of the more general processes by which the rudiments of the organs and tissues of the embryo may be formed out of the layers or cell masses of the gastrula and post-gastrula stages. We shall not attempt to describe here the actual formation of any specific embryonic structure, but rather shall give a brief classification of the more common and important processes, a few of which have already been mentioned earlier in this chapter.

While the morphogenetic processes within the embryo show the greatest diversity and vary almost infinitely in specific details, yet it is possible to include them all under a few heads, when these matters of detail are omitted. Again it should be recalled that we are limiting our description to the Chordata.

The primary condition of morphogenesis is cell multiplication. After each division the daughter cells increase to practically the size of the parent cell; and numerical increase in cells, together with their growth, i.e., cell proliferation, play either a primary or a secondary part in every morphogenetic process. When the process of cell division is quite general throughout the extent of the germ disc or layer, the result is an increase in the thickness or in the extent of the sheet, respectively, when the plane of the cell divisions is in general parallel with, or perpendicular to, the plane of the whole layer. If there should be little or no regularity in the positions of the division planes, the membrane would increase in aU directions (Fig. 166).

Ordinarily, in embryogeny, cell multiplication and growth are more intense in restricted areas of the blastoderm or germ layer. It is convenient then to distinguish between (a) those processes in which the multiplying cells tend to remain associated in the same general region, and (6) other processes where they become more or less separated, either from one another or from their seat of origin. Under the former head we must again distinguish between the results of increase in thickness and in extent. A localized increase in thickness is frequently termed a hvd; buds may project either above (limb bud) or below (Teleostean lens) the free surface where they are formed. If the thickening region is elongated the result may be the formation of a strand or plate of cells, again either a ridge-like structure above the surface of the layer (genital ridge), or a keel-like thickening below the surface (Teleostean nerve cord, in part).

Increase in extent of a localized area frequently involves the obstructive action of the region bounding the area. When this form of growth occurs generally, so that the tendency to extension occurs in every direction from the middle of the area concerned, the result is frequently an arching, either outward or inward. This may take the form of a simple arching as in the Teleostean blastula (Fig. 150, C), or the same process may be carried farther and followed by a constriction near the base. Such processes are very common indeed and are termed tnvagination and evaginaiioTiy according as the growth is below or above the free surface. Simple illustrations of evagination are afforded by the formation of intestinal villi, the rudiments of lung or thymus, and the like; typical invaginations are seen in the formation of theopti^ cup out of the optic lobe, the auditory sac, etc. When this form of growth in extent is limited to certain directions instead of occurring radially, the result is often the formation of a fold which bears somewhat the same relation to the dilation that the strand does to the bud. The fold also may be above the surface of the membrane, forming a sort of arch or hollow ridge« usually bounded by lateral depressions (frog's pronephric duct), or below the surface forming a groove or furrow bordered by lateral elevations (medullary groove) (Fig. 167). In some instances a solid strand may be formed in this way instead of by the umpler process of direct increase in thickness (Teleostean nerve cord in part). When the processes leading to the formation of a groove ar^ continued, the groove may be converted into a closed canal by the approach, apposition, and fusion of the borders (neural tube) (Fig. 167). In those conditions where the proUferating cells become, to some extent, separated either from one another of from the prohferating region itself, we may note first, instances of actual cell migration. This may be either emigration or immigration, according as to whether we fix attention upon the source or the destination of the migratory cells (mesenchyme cells) . Secondary processes of thickening or thinning may accompany these processes. In other cases the movement of cells may be described as rearrangement rather than migration; this may be iUustrated by the formation of mesodermal somites and blood islands (chick).

Fig. 166. — Diagrams of four stages in the formation of an epithelial thickening, several layers of cells deep. From Korschelt and Heider.

One of the common morphogenetic processes is a combination of increase in thickness and cell rearrangement, such as the usual forma tion of the notochord, or the process of delaminatian, which consists in the splitting of a single thickened sheet into two separate layers, either as a whole or in localized areas (formation of mesoderm in the frog, or division of the mesoderm into somatic and splanchnic layers) ; in some instances the initial thickening may not be very apparent^

Occasionally cells of different layers, or of different rudiments, meet and fuse, forming a continuous rudimentary mass (pituitary body) .

Fig. 167. — Diagrams of the formation of the medullary canal or neural tube, in the Vertebrates. From Korschelt and Heider.

Fig. 168. — Diagrams of the formation of the mouth and stomodseum in a typical form. From Korschelt and Heider. ec, ectoderm; md, pharynx: v<2, stomodseum.

Finally we may mention certain morphogenetic processes of a wholly different kind, namely, resorption, and changes in the form and size of cells. From this point of view merely, the process of resorption may be regarded as the reverse of proliferation. Definite rudiments may appear first as spaces thus formed by the gradual dissolution and absorption of cells in certain areas. Thus the oral and anal openings, gill-clefts, etc.^ are usually formed as "perforations" by the resorption of areas where previously separate and continuous layers became united by a process of fusion (Fig. 1*68) • In other cases rudiments once established may gradually disappear, either wholly (tail of tadpole), or in part (pronephros) remaining then as vestigial organs.

Changes in cell form and size chiefly fall under the head of tissue differentiation, or histogenesis, but in a few instances such processes are primarily involved in the formation of rudiments. The development of the eye affords illustrations of these processes; the lens forms as a thickening of the cells on one side, and the thiniiing on the other, of a sac originally formed by invagination, and the optic lobe, at first nearly spherical, flattens and invaginates, one layer becoming thickened as the chief part of the retina (later complicated by cell proliferation) while the other layer forms a thin pigmented layer.

These processes of morphogenetic value are tabulated in the accompanying sunmiary; the arrangement here is merely a convenient one and has no other significance.

It is extremely important to recognize that these morphogenetic processes described above are not merely simple mechanical processes. The arrangement and behavior of the cells in an invaginating or delaminating region are determined by other factors than those of physical resistances, attractions, etc. These events are both in fundamentals and in details to be regarded as active phenomena of a living organism. They are frequently also to be imderstood from the historical or adaptive points of view.

What the precise conditions are which determine the nature of these events, may usually only be conjectured. In some cases they may result from osmotic conditions, absorption of water, etc. But for the most part the nature of the specific stimuli, and the conditions within the layer or cell group, which lead to the definite reactions of rudiment formation are unknown. And in this particular field of development we can do little more than to describe what happens from the morphological viewpoint.

Summary of the Chief Morphoqenetic Processes

I. Cell division and growth throughout the layer or disc, resulting in (a) Increase in thickness. (h) Increase in extent, (c) Increase in both thickness and extent. II. Cell division and growth localized in restricted areas of the layer or disc. A. Cells remain related and in continuity (a) Increasing in thickness.


1. Radially — formation of buds


{Ridge. K 1

(&) Increasing in extent.


1. Radially Invagination.



2. Chiefly in one axis Folds.

Tube (strand secondarily).

B. Cells become separated to a varying degree.

f \ MS' +* / I^™^gr*tion 1 with corresponding

Emigration J thickening and thinning.

(&) Rearrangement.

(c) Delamination (sometimes preceded by thickening).

(d) Fusion.

III. Resorption.

(a) Accompanying a process of perforation. (&) Disappearance (degeneration).

IV. Changes in cell form and size.

(a) Thickening. (6) Thinning.

References to Literature

Cerpontaine, p., Recherches sur le d^veloppement de TAmphioxus. Arch. Biol. 22. 1906.

Davenport, C. B., Studies in Morphogenesis. IV. A preliminary Catalogue of the Processes concerned in Ontogeny. BuU. Mus. Comp. Zool. Harvard Coll. 27. 1896.

Etcleshtmer, a. C, The Formation of the Embryo of Necturus, with Remarks on the Theory of Concrescence. Anat. Anz. 21. 1902. Greil, A.| Ueber die erste Anlage der Gefasse und des Blutes be! olo- und Meroblastiem. Verh. Anat. Gesell., in Anat. Anz. 32. 1908. Hertwiq, 0., Die Lehre von den Keimbl&ttem. Handbuch, etc. I, 1, 1. 1903 (1906).

Hertwig, O., und R., Studies on the Germ Layers. Jena. Zeit. 13-16 (6-9). 1879-1883.

His, W., Untersuchungen tiber die Entwickelung von Knochenfischen, besonders tiber diejenige des Salmens. Zeit. Anat. Entw. 1.

1876. Untersuchungen tiber die Bildung des Knochenfischem bryo. Arch. Anat. Entw. 1878.

Jenkinson, J. W., (Ref. Ch. VII.) .

Keibel, F.y Die Gastrulation und die Keimblattbildung der Wirbeltiere, Ergebnisse Anat. u. Entw. 10. 1900 (1901).

KopscH, F., Untersuchungen tiber Gastrulation und Embryobildung bei den Chordaten. I. Die Morphologische Bedeutung des Keim hautrandes und die Embryobildung bei der Forelle. Leipzig. 1904.

KoBSCHELT UND Heider, Lehrbuch, 6fo. I Abschnitt. Experimentelle Entwicklungsgeschichte. Jena. 1902. Ill Abschnitt. Furchung und Keimblatterbildung. Jena. 1909-1910.

LiLLiB, F. R., The Development of the Chick. New York. 1908.

Patterson, J. T., On Gastrulation and the Origin of the Primitive Streak in the Pigeon's Egg. — ^Preliminary Notice. Biol. Bull. 13. 1907. Prbnant, a., BoxnN, P., bt Maill^rd, L., Traits d'Histologie. Paris. 1911.

ScHAuiNSLAND, H., Beitr&ge zur Entwickelungsgeschichte und Anatomie der Wirbeltiere. I, II, III. Bibliotheca Zoologica. 39. Stuttgart. 1901-1903.

ScHULTZE, 0., Ueber das erste Auftreten der bilateralen Symmetrie im Verlauf der Entwicklung. Arch. mikr. Anat. 55. 1900.

Selys-Lonqchamps, M. de, Gastrulation et formation des feuillets chez Petromyzon planeri. Arch. Biol. 25. . 1910.

Sumner, F. B., KupfFer's Vesicle and its Relation to Gastrulation and Concrescence. Mem. N. Y. Acad. Sci. 2. 1900. A Study of early Fish Development. Experimental and Morphological. Arch. Entw.-Mech. 17. 1903.

WniTBiAN, C. O., The Embryology of Clepsine. Q. J.M.S. la 1878.

Will, L., Beitrage zur Entwicklungsgeschichte der Reptilien. I. Die Anlage der Keimblatter beim Gecko (Plaiydactylus facetanus).

Schreib. Zool. Jahrb. 6. 1892. Wilson, H. V., (Ref. Ch. VI.)

Ziegler, H. E., (Ref. Ch. III.) ZiEOLER, H. E., und F., Beitr&ge zur Entwickelungsgeschichte von Torpedo. Arch. mikr. Anat. 39. 1892.

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Kellicott WE. A Textbook of General Embryology (1913) Henry Holt and Co., New York.

Kellicott (1913): 1 Ontogeny | 2 The cell and cell division | 3 The germ cells and their formation | 4 Maturation | 5 Fertilization | 6 Cleavage | 7 The germ cells and the processes of differentiation, heredity, and sex determination | 8 The blastxtla, gastrula, and germ layers. Morphogenetic processes

Cite this page: Hill, M.A. (2024, April 17) Embryology A textbook of general embryology (1913) 8. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/A_textbook_of_general_embryology_(1913)_8

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