Book - Text-Book of the Embryology of Man and Mammals 5
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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The Development of the Two Primary Germ-Layers (Gastraea-Theory)
THE advances which are brought about during the next stages in the development of the blastula depend primarily upon processes of folding. By these means there arise larval forms, which are at first composed of two, and afterwards of four epithelial membranes, or germ-layers.
The larval form which is composed of two germ-layers is called the gastrula. It possesses an important developmental signification, because, as HAECKEL has shown in his celebrated Gastrsea-Theory, it is to be found in each of the six chief branches of the animal kingdom, and thus furnishes a common starting-point from which along diverging lines the separate animal forms may be derived. As with blastulse, so in the case of the gastrula four different kinds can be distinguished, according to the abundance and the method of distribution of the yolk. Starting from a simple fundamental form, three further modifications have arisen, all of which, with the exception of a single one which is characteristic of many Arthropods, are to be encountered within the phylum of Vertebrates.
The simplest and most primitive form, with the consideration of which we have to begin, is found only in the development of Amphioxus lanceolatus. As has been previously shown, its blastula is composed of cylindrical cells, which are closely joined into a single-layered epithelium (fig. 43). At one place, which may be designated as the vegetative pole (VP), the cells (vz) are somewhat larger and more turbid, owing to the yolk-granules lodged in them. The process of the formation of the gastrula commences at this place. The vegetative surface begins at first to be flattened, and then to be pushed in toward the middle of the sphere. By the advance of the invagination the depression grows deeper and deeper, while the cleavage-cavity becomes to the same degree diminished in size. Finally, the invaginated portion (fig. ik) comes in contact with the inner surface of the uninvaginated portion (ak) of the blastula, and completely the obliterates cavity. As a result there has been formed out of the hollow sphere with a single wall a cup-shaped germ with double walls the gastrula.
Fig. 43. Blastula of Amphioxus lanceolatus, after HATSCHEK. fh, Cleavage-cavity; az, animal cells; vz, vegetative cells; AP, animal pole ; VP, vegetative pole.
Fig. 44. Gastrula of Amphioxus lanceolatus, after HATSCHEK.
ulc, Outer germ-layer ; ik, inner germ-layer ; u, blastopore, or mouth of archenteron (c.d).
The cavity of the gastrula, which results from the invagination and is not to be confounded with the cleavage-cavity which it has supplanted, is the primitive intestine (archenteron) (ud), or the intestinebody cavity (coelenteron). This opens to the outside through the primitive mouth (mouth of the archenteron, blastopore) (u).
Inasmuch as the names primitive intestine and primitive mouth might easily give rise to erroneous conceptions, let it be remarked, in order to preclude from the start such an event, that the cavity and its external opening which arise by this first invagination are not equivalent to the intestine and mouth of the adult animal. The archenteron of the germ, it is true, furnishes the fundament for the intestinal tube, but there are also formed out of it a number of other organs, the chief of which are the subsequently formed thoracic and body cavities. The future destination of the cavity will therefore be better expressed by the term " codenteron." Finally, the primitive mouth is only an evanescent structure among vertebrated animals ; later it is closed and disappears without leaving a trace, while the permanent or secondary mouth is an entirely new structure.
The two cell-layers of the cup, which are continuous with each other at the edge of the blastopore, are called the two primary germ-layers, and are distinguished according to their positions as the outer (ak) and the inner (i&). Whereas in the blastula the individual cells differ only a little from one another, with the process of gastrulation a division of labor begins to assert itself, a fact which may be recognised in the case of the free-swimming larvae of Invertebrates. The outer germ-layer (ak] (also called ectoblast or ectoderm] serves as a covering for the body, is at the same time the organ of sensation, and effects locomotion when cilia are developed from the cells, as is the case with Amphioxus. The inner germ-layer (ik] (entoblast or entoderm) lines the crelenteron and provides for nutrition. The cell-layers thus stand in contrast to each other both as regards position and function, since each has assumed a special duty. In view of this fact they have been designated by C. E. VON BAER as the two primitive organs of the animal body. They present us with a very instructive, because very simple, illustration of the manner in which two organs originate from a single fundament. By invagination the midifferentiated cells of the surface of the blastula are brought into different relations to the outer world, and have consequently been compelled to follow different courses in their development, and to adapt themselves to special duties corresponding to the new relations.
The separation of the embryonic cell-material into the two primitive organs of VON BAER is of decisive significance for the whole subsequent course of the development of the individual cells. For a very definite portion of. all the ultimate organs of the body is referable to each of the two primitive organs. In order to put this important condition in the proper light at once, let it be stated that the outer germ-layer furnishes the epithelial covering of the body, the epidermis with the glands and hair, the fundament of the nervous system, and that part of the sense organs which is functionally most important. On this account the older embryologists imposed upon it the name of dermo-sensory layer. The inner germ-layer, on the contrary, is converted into the remaining organs of the body into the intestine with its glands, into the body-cavity, into the muscles, etc. ; by far the greater mass of the body, therefore, is differentiated out of it, and it has to pass through the most numerous and the most trenchant metamorphoses.*
- The practice of distinguishing the outer and the inner germ-layers as animal and vegetative, which was formerly in vogue and is followed even now, is not proper, and ought therefore to be given up. For the transversely striped musculature of the body, which belongs to its animal organs, does not arise from
Larval forms quite like that of Amphioxus have also been observed in the case of Invertebrates belonging to the phyla of Ccelenterata, Echinodermata, Vermes, and Brachiopoda. For the most part they quit the egg-envelope, even in the gastrula stage, to swim about in the water by means of their cilia ; and they can now take nutritive substances small infusoria, algse, or remnants of larger animals through the primitive mouth into the digestive cavity, and make use of them in the further growth of their bodies. Likewise the substances which are not serviceable because indigestible are ejected from the body through the same orifice. In the case of the higher animals the ingestion of food is not only impossible at this time, but also superfluous, because the egg and the embryonic cells arising from it still contain yolk-granule?, which are gradually consumed.
Fig. 45. Blastula of Tritontaeniatus. fk, Cleavage-cavity ; dz, yolk -cells ; rz, marginal zone.
The modifications which gastrulation undergoes in the Amphibia are easily referable to the simpler conditions in Amphioxus. In the case of the Water-Salamander, which is to serve as an illustration in this description, one half of the blastula (fig. 45), which is called the animal half, is thin-walled and composed of small cells, which lie in two or three layers one above another, and in the case of the Frog contain black pigment. The other, or vegetative half (dz), exhibits a greatly thickened wall, composed of much larger, more deutoplasmic, polygonal cells (dz), which, loosely associated in several layers, cause a protuberance into the cavity (f/i) of the blastula, which is proportionally diminished in size. Where the differentiated halves meet, a transition is effected by means of cells, forming what GOETTE has designated margined zone (rz). Inasmuch as the specific gravity of the animal half is much less than that of the opposite half, it is without exception directed upward in water. The former the outer germ-layer, as, in consequence of false observations, was formerly believed, but rather from the primary inner germ-layer, as has now been established by many observations.
Fig. 46. Egg of Triton, which is developing into a gastrula, seen from the surface.
u, Primitive mouth (blastopore).
constitutes the thinner roof, the latter the highly thickened floor, of the excentrically placed cleavage-cavity.
When the gastrula begins to be developed, the invagination takes place 011 one side in the marginal zone (fig. 46 u\ and is distinguishable externally by means of a sharp, afterwards horseshoe-shaped furrow, which is bounded on one side by small cells, which in the case of the Frog contain black pigment, on the other side by large unpigmented elements. At the fissure-like blastopore there are infolded into the interior of the blastula (fig. 47 u} along its dorsal lip (ell] small cells, along its ventral lip (vl) the large deutoplasmic elements of the vegetative half ; the former constitute the roof, the latter the floor, of the coelenteroii (ud).
The latter appears in the first stages of the invagination simply as a narrow fissure alongside the capacious cleavage-cavity (fh) ; soon, however, it causes a complete obliteration of this cavity, the fundus of the becoming a broad sac, while the entrance always remains narrow and fissure-like. Since the cceleiiteron of the Amphibia was first observed by the Italian investigator, KUSCONI, it is ordinarily mentioned in the older writings as KUSCONI'S digestive cavity, and the blastopore likewise as the lluscoNiAN anus.
At the close of the process of invagination the whole yolk-mass, or the vegetative half of the blastula, has been taken into the interior to form the lining of the ccelenteron, being at the same time overgrown by a layer of small cells (fig. 48). In the case of the Frog the invagination enlarged into whole surface of the germ, with the exception of a small place about as large as the head of a pin, which corresponds to the blastopore, now appears black, because the small cells are deeply pigmented. At the place except ed a part of the unpigmeiited yolk-mass protrudes through the blastopore and closes the entrance to it as if with a stopper (cZ), by reason of which it bears the significant name of vitelline plug.
Fig. 47. Longitudinal [sagittal] section through an egg of Triton at the beginning of gastrulation.
ak, Outer germ-layer ; Ik, inner germ-layer ; fh, cleavagecavity ; iid, ccelenteron ; u, blastopore; ilz, yolkcells ; dl and 'd, dorsal and ventral lips of the ccelenteron.
Of the two germ-layers of the gastrula the outer subsequently becomes reduced in thickness in the case of the Water-Salamander to a single layer of regularly arranged cylindrical cells, whereas in the case of the Frog it is composed of two or three layers of small, in part cubical, deeply pigmented elements. The inner germ-layer in the roof of the coeleiiteron likewise consists of small (in the Frog, pigmented) cells, but in the floor it is composed of large yolk-cells, which, heaped together in many layers, produce an elevation that projects far into the ccelenteron and partly fills it. For this reason the gastrula in Amphibia is compelled to adopt in water a definite position of rest, because the yolk-mass, being the heavier part, always assumes the lowest position (fig. 48).
Fig. 48. Sagittal section through an egg of Triton after the end of gastrulation. ak, ik, d-, <//, <:l, ud, as in fig. 47; d, vitelline plug; mk, middle germ-layer.
The germ of the Amphibia is already a bilaterally symmetrical body. The thickened, yolk-containing wall of the gastrula becomes the ventral side of the adult animal ; the opposite wall, or roof of the ccelenteron, becomes the dorsum. The blastopore indicates, as the sequel shows, the posterior end, the opposite part the head-end. There may therefore be passed through the gastrula a longitudinal, a dorso-ventral, and a transverse axis, which correspond with the axes of the adult animal. This bilateral symmetry, which appears so early in the Amphibia, is solely attributable to the accumulation of yolk-material, and to the piling up of it on the ventral side of the ccelenteron.
The development of Amphibia furnishes us with a transitional condition, which is serviceable for the comprehension of the much more highly altered form 1 which the gastrula acquires in the case of eggs with partial cleavage in the classes of Selackiij Teleosts, Reptiles, and Birds.
The conditions are the most readily intelligible in the case of the Selachians. That which we have described in the blastula of the Amphibia as the roof of the cleavage-cavity is in the blastula of the Selachians a v dk kz dk H small disc of embryonic cells (fig. 49 kz), continuous at its margin with the extraordinarily voluminous yolk - mass (dK), which contains nuclei, although it is not divided up into cells. This yolk-mass corresponds to the yolk-cells of the Amphibia, and, like the latter, forms the floor of the cleavage-cavity Germ-disc and yolk thus together constitute a sac with an almost obliterated cavity (B\ and with walls differing in thickness and in differentiation. A very small part of the wall, the germ-disc, consists of cells. The much larger and thicker portion is yolk-mass, which in the vicinity of the cavity contains nuclei, but is not divided into cells.
Fig. 50. Median section through a germ-disc of Pristiurus, in which the gastrular invagination has begun, after RUCKERT. ud, First rudiment of the coelenteron ; B, cleavage-cavity ; dk, yolk-nuclei ; fd, finely granular yolk ; gd, coarsely granular yolk ; V arid H, front and hind margins of the germ-disc.
As in the Amphibia, so here, the gastrulation begins at what is subsequently the hind end (H] of the embryo, at a region in the zone of transition or margin of the germ-disc, in which the most superficial cells have assumed the cylindrical form, and are closely joined together (fig. 49). The margin of the disc is folded in (fig. 50) toward the cleavage-cavity (/>), so that a small ccelenteron (ud), shown in the accompanying section, and a fissurelike blastopore are distinctly recognisable. The neighboring yolk also participates in the invagination, since in the territory of the zone of transition the yolk-nuclei (dk), enveloped in protoplasm, become detached from the yolk, grow into the cleavage-cavity along with the invaginated cells, and contribute to the formation of the inner germ-layer in a similar manner to that in which, in the case of the Amphibia, the vegetative cells at the lower lip of the blastopore are carried in with the invagination into the cleavage-cavity. The cleavage-cavity (Z?) is being continually encroached upon by the ingrowth of the cells originally in its roof, which form a continuous layer projecting from behind forward. Consequently in the Selachians also the germ-disc becomes two-layered as the result of the invagination. It lies so close upon the yolk, that the crelenteron appears at most as a fissure. Moreover, the invagination in the Selachians does not remain limited to one region of the original margin of the germ-disc, but soon stretches itself out over its whole posterior perimeter. The blastopore then appears as a large semicircular or horseshoe-shaped fissure at the future posterior end of the embryonic fundament.
The enormous thickness of the yolk causes an important difference between the gastrulatioii of the Selachii and that of the Amphibia. In the case of the latter the mass of the yolk-cells w r as quite rapidly carried in with the invagination, and employed in the formation of the ventral wall of the ccelenteroii. In the Selachians the taking up of the yolk into the interior of the body ensues only at a slow rate (in a manner to be more accurately explained later), so that for a long time only the dorsal side of the gastrula consists of two celllayers, whereas the ventral wall is formed by the yolk-mass.
The eggs of Teleosts are very nearly related to those of Selachians in their whole method of development. The same cannot be said to be true to the same extent for the eggs of Reptiles and Birds. The latter, indeed, also belong to the meroblastic type, since they have developed a large amount of yolk, and in consequence undergo partial segmentation ; but in the formation of the germlayers, they exhibit many peculiarities, so that they require a separate treatment. In Birds and Reptiles the investigation is accompanied with greater difficulties than in the Selachians. Particularly the development of the germ-layers in the Chick, notwithstanding the fact that the best investigators have given it their attention, has for a long time been the subject of very divergent descriptions. At the present moment, however, the main facts in the case have been established for the Bird's egg also by the very recent and excellent work of DUVAL, and upon this as a basis the gastrulatiori in Birds is easily to be correlated with that of the Vertebrates hitherto described. Since the Bird's egg has played such an important role in the history of embryology, and has even been called a classical object for investigation, it appears necessary to go briefly into the conditions which it presents in the gastrula-stage, and in connection therewith to consider some of the important results drawn from the study of the eggs of Reptiles.
The blastula arises and the germ-layers begin to be developed out of it while the Bird's egg tarries in the terminal region of the oviduct.
The blastula arises in a manner which was first correctly described by DUVAL. When by the process of segmentation a small disc of cells has been formed, there appears in the latter a narrow fissure, the cleavage-cavity (fig. 51 f/i), and the cellmaterial is separated into an upper layer (dw} and a lower layer (vw}, which are continuous with each other at the margin of the disc. The upper layer consists of fully isolated cleavagespheres, which are flattened at their surfaces of contact and arranged into an epithelium-like layer. They correspond to the thin-walled half of the blastula in Triton (fig. 45), which has already been designated as the animal half. The lower layer is composed of larger cleavage-spheres, which are still in great part continuous by means of their lower halves with the white yolk (wd), which is spread out beneath the germ-disc and is known as PANDER'S nucleus. Yolk-nuclei (merocytes) are also found here in great numbers, especially around the whole periphery of the germ-disc. Since they increase in number by nuclear division, and since some of them, enveloped in protoplasm, become detached from the yolk, they contribute to the continuous growth of the germ-disc, a process which has already (p. 65) been described as supplementary cleavage. The lower cell-layer, together with the whole yolk-mass with its free nuclei, must be compared to the vegetative half of the blastula of Triton (fig. 45 dz).
Fig. 51. Section through the germ-disc of a freshly laid unfertilised Hen's egg, after DUVAL. fh, Cleavage-cavity ; v;d, white yolk ; rw, lower cell-layer ; dw, upper cell-layer of the blastula.
The gastrulation proceeds from the posterior margin of the germdisc, and begins even some time before the egg is laid. The study of it is coupled with great difficulties, and demands, most of all, that, in the investigation of the disc by means of sections, one should be accurately informed concerning the position of its anterior and posterior margins. The orientation is essentially facilitated by the fact that, in the case of every Hen's egg, with rare exceptions, the side toward which the front end of the embryo is directed can be stated accurately before opening the shell. This results from the following rule established by KUPFFER, KOLLER, GERLACH, and DUVAL.
When one so places an egg in front of him that the blunt pole is turned to the left, the more pointed one to the right, then a line uniting the two poles divides the germ-disc into a half on the side toward the observer, which becomes the hind end of the embryo, and a forward half, which is developed into the head-end. By taking into account this rule, one can establish a difference on the germdisc even during the process of cleavage. In the anterior region the cleavage takes place more slowly than in the posterior half. Consequently larger embryonic cells are found in front, smaller and more numerous ones behind (OELLACHER, KOLLIKER, DUVAL).
The difference between anterior and posterior becomes more evident at the beginning of gastrulation. If one now examines carefully the thickened margin of the germ-disc (Randwulst of German writers, bourrelet blastodermique of DUVAL), it is seen that the disc is limited in front and on the sides by a notched and indistinct boundary, but behind, 011 the contrary, by a sharper contour. The latter is caused by the fact that the marginal ridge, in consequence of a more vigorous growth of the cells, has become thickened and more opaque, and has assumed a whiter colour. It is distinctly recognisable from its surroundings as a whitish cresceiitic figure (fig. 52 A s). Often there is also observable in the crescent a narrow furrow, the crescentic groove (Sichelrinne, KOLLER), by means of which the germdisc acquires a still sharper limitation behind.
DUVAL has proved by means of sections, part of which was made in a transverse direction, and part in the sagittal, that the Bird's egg is now in the gastrula stage. Especially instructive are the two median sections, figs. 53 and 54. As is to be seen at once in fig. 53, which represents the somewhat younger stage, the crescentic groove described as occupying the posterior part of the marginal ridge (vl) is continued in the form of a narrow fissure (ud). Whereas in the blastula stage (fig. 51) the lower celllayer passed over continuously into the white yolk, it is now sharply separated from it as far as the fissure extends. In fig. 53 this separation has been completed only in the posterior half of the germ -disc ; in the
Fig. 52 A. The unincubated germ-disc of a Hen's egg, after ROLLER.
d, Yolk ; ksck, germ-disc ; s, crescent ; V and H, anterior and posterior margins of the germ-disc.
B. The germ-disc of a Hen's egg during the first hours of incubation, after ROLLER. d, Yolk ; ksch, germ-disc ; Es, embryonal shield ; s, crescent ; sk, knob of the crescent ; V and //, anterior and posterior margins of the germ-disc.
Fig. 53. Longitudinal section through the germ-disc of an anterior half Oil the CO11 unincubated egg of the Siskin (Carduelis spinus), after DUVAL.
ak, Outer , ik, inner germ-layer ; wd, white yolk ; ilk, yolknuclei ; ud, ccelenteron ; vl, anterior lip, hi, posterior lip at the place of invagination (crescentic groove or blastopore).
trary, embryonic cells (dk) and yolk are still continuous. However, in the somewhat older stage (fig. 54) the connection is terminated in this region also, since the fissure (ud) has extended itself nearly to the anterior margin of the disc (vr). In consequence of this process the part of the white yolk which lies beneath the fissure has become destitute of cells and nuclei, with the exception of the marginal territory, where, especially behind (hi) the crescentic groove, free nuclei are constantly to be found keeping up the supplementary cleavage.
Owing to the appearance of the new fissure (subgerminal cavity) (fig. 53 ud), the cleavage-cavity (fig. 51 fh) is almost completely obliterated. The two cell-layers of the blastula-stage (fig. 51 dtv, vw), described as lying one above and one below the cleavagecavity, have come close together (figs. 53 and 54), being separated from each other by only a narrow fissure. In the upper layer (ak) the cells have assumed a cubical, and at a somewhat later stage a cylindrical, form, and constitute a compact epithelial membrane. The lower layer (ik) is composed of larger roundish and loosely arranged cells in several layers. The former is the primary outer germ -layer, the latter the inner layer. In the region of the posterior marginal ridge (vl), where the cells are at the same time engaged in more active proliferation, the two layers are continuous with each other.
The highly important processes, by means of which are produced the conditions represented in figs. 53 and 54, present many points of comparison with the gastrulation of the Selachians and Amphibia. We can conceive that the newly appearing fissure has arisen, as in the case of the germ-disc of Pristiurus (fig. 50), by an infolding, in such a way that, as in the former case, cells grow inward from the posterior marginal ridge ; and that] at the same time, at the deep part of the invagination, the cells which are originally continuous with the yolk (fig. 53 dk) detach themselves from the latter, and are employed for the increase of the inner germ- layer.
If this explanation is correct, the fissure (ud) which now exists between the inner germ-layer and the floor of the yolk corresponds to the coelenteron, as GOETTE and RAUBER have already remarked, and as DUVAL has for the first time demonstrated ; moreover, the crescentic groove (fig. 52 s) corresponds to the blastopore ; the thickened portion of the marginal ridge (fig. 53 vl) which lies in front of the crescentic groove, within whose territory the two primary germlayers are continuous with each other, is the anterior or dorsal lip of the blastopore ; and the yolk (hi) which lies behind the crescentic groove, and which at this early stage contains numerous free nuclei, may be designated as the posterior or ventral lip of the blastopore.
The development of the crelenteron. is the cause of the gradual reduction of the cleavage - cavity, and of its persisting only as a narrow fissure separating the primary germ-layers.
Fig. 55 Embryonic fundament of Lacerta agilis, after KUPFFER. hf, Area pellucida ; <//, area opaca ; u, blastopore; , crescent; es, embryonic shield. V, anterior, H, posterior end.
The points of c o m p a r i s o n with the gastrula of Triton (fig. 47) are made evident as soon as we
HiaSS of yolk cells with unsegmented yolk, and imagine nuclei imbedded in the latter in the region of the ventral lip of the blastopore.
Through the exposition, given by DUVAL, it appears to me that the contest concerning the origin of the two primary germ-layers in Birds has been happily settled. For a long time there have existed on this vej^i question two irreconcilable views.
According to the older view, to which many investigators still cling, the germ-disc which results from the process of cleavage is divided by fission into an upper and a lower layer (PANDER, VON BAER, REMAK, KOLLIKER, His, and others). According to the other one (HAECKEL, GOETTE, RAUBER, DUVAL, and others), the lower layer has arisen by an infolding. Only by means of the theory of infolding can be explained the different conditions of the anterior and posterior margins of the germ-disc, the more active cell-growth in the territory of the crescent, the existence of a crescentic groove, and the continuity of the two primary germ -layers which is demonstrable in that region. Only by means of this theory, finally, is the relation of Birds to the lower classes of the Vertebrates made possible.
The discoveries which KUPFFER UND BENECKE have made in their investigations of Reptiles, which are so closely related to Birds, also contribute to the elucidation of the pending controversy. In the case of Lacerta agilis (fig. 55), Ernys europsea, etc., there is found, as in the case of the Hen at a corresponding stage of development, at the boundary of the pellucid and opaque areas of the posterior end of the germ-disc, an exuberant cell-growth in the form of a crescent (s). In the middle plane and slightly in front of this crescent there is to be seen a small, transversely placed, fissure-like opening (tt), ivhicli leads into a blind sac and is comparable to the crescentic groove. KUPFFER rightly interprets the opening as the blastopore, which is enclosed between an anterior and a posterior lip, and the cavity as 'the ccelenteron. He also draws a comparison between the corresponding structures in Birds and Reptiles.* Let us now direct our attention to the succeeding developmental stages of the germ-disc of the Chick. These consist, chiefly, in a constant increase of the superficial extent of the disc.
In the freshly laid, unincubated egg (fig. 54) the outer germ-layer (ak) is composed of a single sheet of closely united cylindrical cells ; the inner layer (/&), on the contrary, consists of a two-layered to three-layered bed of somewhat flattened elements, which are only loosely associated.
Under the influence of incubation the superficial extension of the germ-disc makes rapid advances (fig. 56). In this process the outer germ-layer (ak) outstrips the inner, and terminates in a region of the yolk where the latter has not yet undergone division into entoderinic cells. In the form of its cells it is, in every respect, in sharp contrast with the inner layer. While the ectodermic cells (fig. 56 ak] attain their greatest height in the middle of the germ-disc, they gradually decrease in height toward the margin, and undergo a transition into cubical and finally into flattened elements (fig. 57). The reverse is the case with the inner germlayer ; the latter has now become converted in the middle of the germdisc (fig. 56 ik) into a single layer of much flattened scale-like cells, which are closely united into a thin membrane. Toward the periphery they become somewhat larger and more polygonal (fig. 57), and here, at some distance inside the free margin of the outer germ-layer, they become merged in the white yolk (dw), which is abundantly provided with yolk-nuclei (dfy in the region of the transition. This region of the yolk is designated as the yolk-wall (vitelline rampart). It serves for the augmentation of the inner germ-layer, in that the free nuclei increase in number by division, and keep up the process of supplementary cleavage already mentioned.
- in the interpretation of the manner in which the imagination takes place in the case of the eggs of Reptiles and Birds, I differ from other investigators who also maintain that a gastrulation. takes place (GoETTE, HAECKEL, RAUBER, BALFOUR, and others). They regard the whole margin of the germdisc as the blastopore, at which the outer germ-layer bends over to become continuous with the inner layer. According to my interpretation, the invagination occurs at a small circumscribed place of the margin. The blastopore is from the beginning surrounded by cells both on its anterior and its posterior lip. The relation of the blastopore as well as that of the germ-layers to the yolk will be more fully dealt with hereafter.
During incubation the liquefaction of the yolk makes further progress (fig. 56) and leads to the formation of a depression (ud), which continually increases in depth and breadth, and over which the germdisc arches like a watch-glass. Upon examination from the surface its middle, as far as the fluid reaches under it, appears clear and translucent, whereas the marginal area, which lies upon the opaque yolk, appears dark. Such a distinction is still more observable when one detaches the
w 7 hole germ-disc from the yolk, f^^ * for in the region of the O^^-x^^e^^^^^^v.^^^-^w^^ dk fluid-filled space the thin and transparent germ- ^"'^^ *jx? dw layers come off easily and ||| clean from their substratum wlipvpas it the vim ^ig. 57. -Section through the margin of the germ-disc n > WJ of a Hen's egg that had beep incubated for six from the point where the hours, after DUYAL.
, ak, Outer germ-layer; J.:, yolk-cells; elk. yolk-miiclei ; inner germ-layer merges ,,. yolk . wall . with the yolk-wall outward, turbid yolk-substance remains clinging to the germ-disc. For a long time the middle, clear, circular area has been designated in embryology as the clear germinal area (area pellucida), and the more cloudy, ring- like rim as the opaque germinal area (area opaca).
In the next chapter I shall treat more in extenso of the important changes which take place up to the time when the egg is laid and during the first hours of incubation in the vicinity of the crescentic groove and the anterior lip of the blastopore, because they are connected with the development of the middle germ-layer.
It is still more difficult than in the case of the Chick to interpret in its details the development of the germ-layers in Mammals, and to refer it back to the gastrulation of the other Vertebrates. Especial service has been rendered through the painstaking investigation of these conditions : in the earlier times by BISCHOFF, in later years by HENSEN, LIEBERKUHN, VAN BENEDEN, KOLLIKER, and HEAPE. The object of investigation which has been made use of in this work, and which we shall employ as the basis of our description, has usually been the Rabbit ; besides this, the Bat and the Mole have also been employed,
While llit 1 Mammalian egg is gradually impelled through the oviduct toward the uterus by the ciliary motion of the epithelium, it becomes converted by the cleavage process into a spherical mass of small cells (fig. 58 A). Then there arises within it, by the secretion of a fluid, a small fissure-like cleavage-cavity (fig. 58 B). The germ has consequently entered upon the vesicular or bias tula stage. The wall of the blastula, or vesicula blastodermica, is composed of a single layer of polygonal cells, arranged, as has been known since BISCHOFF'S works, in mosaic, with the exception of a small region, where the wall, as in the case of the Amphibian blastula, is thickened by an accumulation of somewhat more granular and darker cells, which produce a knob-like elevation that projects far into the cleavage-cavity.
Fig. 58. Optical sections of a Rabbit's egg in two stages immediately following cleavage, after ED. v. BENEDEN. Copied from BALFOUR'S "Comparative Embryology." A , Solid cell-mass resulting from cleavage.
^'Development of the blastula by the formation of a cleavage-cavity in the cell-mass. (According to VAN BENEDEN'S interpretation, ep is epiblast ; liy, hypoblast ; bp, blastopore.)
A peculiarity preeminently characteristic of the further development of Mammals is that here, as in 110 other Vertebrate, the blastula increases enormously in size (fig. 59), by the accumulation of fluid which contains much albumen and produces a granular coagulum upon the addition of alcohol ; it soon acquires a diameter of I'O mm. Of course, with these processes of growth the zona pellucida is altered and distended into a thin membrane. A gelatinous layer (zp] already secreted by the oviduct envelops the latter.
In Rabbits' eggs which are a millimetre in diameter the wall of the blastula has become very thin. The mosaic-like cells arranged in a single layer have become very much flattened. Also the knob of cells, which projects into the cleavage-cavity, has become metamorphosed and has spread itself out more and more in the form of a disc-like plate, which is continuous at its attenuated margins with the thin wall of the blastula. The further processes of development take place principally in this plate. Its most superficial cells are flattened out to thin scales, such as also form the wall of the blastula elsewhere ; its remaining elements, on the contrary, arranged in from two to three superposed layers, are larger and richer in protoplasm.
Up to this time the embryo of the Mammal is in the blastula stage. It still consists everywhere of a single germ-layer. For the view which has been advanced by many persons, that the germ-disc in this stage of development is already in the two-layered condition, and that the outer layer of flat cells constitutes the outer germ-layer and the more protoplasmic cells lying under it the inner germ-layer, is, in my opinion, untenable. Opposed to this are, first, the fact that the flattened and the thicker cell-layers are firmly joined together and are not separated from each other even by the narrowest fissure, and, secondly, the further course of the development.*
Fig. 59. Rabbits egg, 70-90 hours after fertilisation, after ED. v. BKNEDEX. Copied from BALFOUR'S " Comparative Embryology." fir. Cavity of the blastula ; ~.p, [gelatinous layer surrounding the] zona pellucida ; <-p, Inj, as in Fig. 58.
Fig. 60. Cross section through the almost circular germinal area of a Rabbit' s egg 6 days and 9 hours old (diameter 0'8 mm*), after BALFOUR. ak, Outer, He, inner germ-layer. The section shows the peculiar character of the upper layer with a certain number of flattened superficial cells. Only about half of the whole breadth of the germinal area is repressnted. +++++++++++++++++++++++++++++++++++++++++
- Holding to this interpretation, I am of course also unable to agree with a view of VAN BENEDEN'S, according to which the gastrulation takes place at the
end of the first stages of cleavage. He interprets in the originally solid sphere of cells (fig. 58 J.) the darker and larger centrally located elements (Jiy} as entoderm, the layer of smaller and clearer cells (^;) surrounding the latter as ectoderm, and a small vacuity in this investing layer as the blastopore (jbp). I, on the contrary, believe that the gastrulation takes place in the manner described on page 104.
Two germ-layers first appeal- in which have already attained a diameter of more than 1 mm. and are about five days old. At the place where the cell-plate previously lay, one sees by inspection from the surface a whitish spot, which is at first round, but later becomes oval or pear-shaped. It is generally designated at this stage as area embryonalis, or as embryonic spot. It consists of two germ-layers (fig. 60), which are separated by a distinct fissure, and may be detached from each other. The inner germ-layer (ik} is a single sheet of greatly flattened cells. The outer germ-layer (ak), on the contrary, is considerably thicker, and shows that it is composed of two sheets of cells : (1) a deeper layer of cubical or roundish, larger elements, and (2) a superficial layer of isolated flatter cells, which were first accurately described by RAUBER, and which have been named after him RAUBER'S layer. Toward the margins of the embryonic spot the outer layer becomes thinner and possesses only a single layer of cells ; these are continuous with the large flattened elements which, as we have seen, alone constitute the greater part of the wall of the sac in the blastula stage. The inner germ-layer is at first developed on only a small part of the wall of the sac at the embryonic spot and its immediate vicinity ; it terminates with a free notched margin, where there are to be found loosely associated amoeboid cells, which by their increase in number and migration probably cause the further growth of the layer. This on older eggs slowly spreads itself from the embryonic spot toward the opposite pole, and thereby the whole blastodermic vesicle gradually become^ two-layered. While this is taking place, changes also proceed at the embryonic spot, which has become oval and somewhat larger. RAUBER'S layer disappears * (fig. 61) ; the underlying cubical or spherical cells have become cylindrical and more closely crowded together. Each of the primary germ-layers is now composed of a single layer of cells.
The two accompanying figures, which represent in two different positions a Rabbit's egg seven days old, will serve for the illustration of these conditions. In looking down from above (fig. 62 A) one sees the embryonic spot (ag), now become oval. It is produced exclusively by a definitely limited thickening of the outer germ-layer, and indicates the place at which the cells are cylindrical ; in that respect it corresponds to the embryonic shield of reptilian and avian embryos, and is not to be confounded with the cell-plate (fig. 59), which was described as a thickening of the one-layered blastula. In looking at it from the side (fig. 62 B] one can distinguish on the blastula three regions : (1) the embryonic spot (ay)', (2) a region which includes the upper half of the vesicle and reaches to the line ge, in which the wall is still composed of two layers, but in which the cells of both the outer and inner germ-layers are very much flattened ; and (3) a third portion lying below the line ge, where the wall is composed exclusively of the outer germ-layer.
There now arises the important question, in what manner the twolayered condition in Mammals arises out of the single-layered form. One has reason to expect that gastrulation takes place here in the same way as with the remaining Vertebrates, by means of an invagination or an ingression of cells which proceeds from a definite territory of the thickened cell-plate of the blastula ; in this connection attention must be directed to the posterior end of the embryonic spot.
When the embryonic spot has acquired a pear-shaped appearance (fig. 63), there is at its posterior end a somewhat less transparent, because thickened, place (Jiw), which KOLLIKER has designated the terminal ridge (Endwulst). It is comparable with the opacity at the posterior margin of the germ-disc of Reptiles and Birds, when their gastrulation begins. An imagination proceeding from this point, such as DUVAL has established for the Chick, is unfortunately not as yet proven with sufficient certainty in the case of Mammals ; the origin of the two-layered stage is also still involved in obscurity.
- Two views are held concerning the manner in which KAUBER'S layer disappears. According to BALPOUR and HEAPE, the flat cells become metamorphosed into cylindrical cells, which are interposed between the other cylindrical cells ; according to KOLLIKER, on 'the contrary, they disintegrate and disappear.
However, there are in the literature some observations, which, fragmentary as they are, appear to mo to be worthy of special regard.
At the stage at which the blast ula has become for a certain distance twolayered (fig. 62), there has been discovered by HEAPE in the case of the Mole, by SELENKA in the Opossum, and by KEIBEL in the Rabbit, at one place of the embryonic spot (probably in the region just described as terminal ridge), a small opening (fig. 64 u), wJiich is possibly to be interpreted as blastopore and to be compared with the crescentic groove of Birds. Here the two primary germ layers are continuous with each other, and from here, as well as from the primitive streak, the middle germ-layer takes its origin. I assume that, beginning at this place, the lower germ-layer has in a still earlier stage been developed by an infolding of a small territory of the single-layered blastula (fig. 59).
Fig. 62. Blastula of the Rabbit 7 days old without the outer egg-membranes. Length 4'4 mm. After KOLLIKER. Magnified 10 diameters.
Seen in A from above, in B from the side.
ay, Embryonic spot (area embryonalis) ; ge, the line up to which the blastula is two-layered.
One circumstance is especially characteristic of the gastrulation of Mammals : that the invaginating membrane is not a closed blind sac, but possesses a free margin, with which it grows along on the inner surface of the outer germ-layer, until it has completely lined the blastodermic vesicle. The reader will please compare with this the statements on page 102. But the absence of a ventral closure becomes intelligible, when we imagine that the yolk-mass, which constitutes in nieroblastic eggs or in Amphibian eggs the floor of the ccelenteron, has degenerated and wholly disappeared. In this case ccelenteron and cleavage-cavity become one and the same, as is the case with Mammals.
Moreover we are induced to assume that in the eggs of Mammals a regressive metamorphosis of originally abundant yolk-contents must have taken place, on account of many phenomena in their development, which would be unintelligible without this assumption. These phenomena will be considered more at length in a subsequent chapter.
Fig. 63. Pear-shaped embryonic spot of a Rabbit's egg 6 days and 18 hours old, after KOLLIKER.
pg, Short primitive streak ; hu\ crescentshaped terminal ridge ; V, anterior, H, posterior end.
Fig. 64. Median section of the embryonic fundament of a Mole's egg through that part in which the primitive streak has begun to be formed, after HEAPE. u, Blastopore ; ul-, outer, ik, inner germ-layer ; V, anterior, H, posterior end.
Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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