The germinal wall at this stage corresponds in many respects with the granular material, forming a ring below the edge of the blastoderm in Teleostei.
It retains the characters above enumerated till near the close of the first day of incubation, i.e. till several mesoblastic somites have become established. It then becomes more distinctly separated from the subjacent yolk, and its component parts change very considerably in character. The whole wall becomes much less granular. It is then mainly formed of large vesicles, which often assume a palisade-like arrangement, and contain granular balls, spherules of white yolk, and in an early stage a good deal of granular matter (vide [fig. 115]). These bodies have some resemblance to cells, and have been regarded as such by Kölliker (No. [135]) and Virchow (No. [150]): they contain however nothing which can be considered as a nucleus. Between them however nuclei[62] may easily be seen in specimens hardened in picric acid, and stained with hæmatoxylin (these nuclei are not shewn in [fig. 115]). These nuclei are about the same size as those of the hypoblast cells, and are surrounded by a thin layer of granular protoplasm, which is continuous with a mesh-work of granular protoplasm enveloping the above described vesicles. The germinal wall is still continuous with the hypoblast at its edge; and close to the junction of the two the hypoblast at first forms a layer of moderately columnar cells, one or two deep and directly continuous with the germinal wall, and at a later period usually consists of a mass of rounder cells lying above the somewhat abrupt inner edge of the germinal wall.
The germinal wall certainly gives rise to the hypoblast cells, which mainly grow at its expense. They arise at the edge of the area pellucida, and when first formed are markedly columnar, and enclose in their protoplasm one of the smaller vesicles of the germinal wall.
In the later stages (fourth day and onwards) the whole germinal wall is stated to break up into columnar hypoblast cells, each of them mainly formed of one of the vesicles just spoken of. After the commencing formation of the embryo the mesoblast becomes established at the inner edge of the area opaca, between the germinal wall and the epiblast; and gives rise to the tissue which eventually forms the area vasculosa. It seems probable that the mesoblast in this situation is mainly derived from cells formed around the nuclei of the germinal wall, which are usually specially aggregated close below the epiblast. Disse (No. [122]) has especially brought evidence in favour of this view, and my own observations also support it.
The mesoblastic somites begin to be formed in the lateral plates of the mesoblast before the closure of the medullary folds. The first somite arises close to the foremost extremity of the primitive streak, but the next is stated to arise in front of this, so that the first formed somite corresponds to the second permanent vertebra[63]. The region of the embryo in front of the second formed somite—at first the largest part of the embryo—is the cephalic region. The somites following the second are formed in the regular manner, from before backwards, out of the unsegmented posterior part of the embryo, which rapidly grows in length to supply the necessary material ([fig. 103]). As the somites retain during the early stages of development an approximately constant breadth, their number is a fair test of the length of the trunk. With the growth of the embryo the primitive streak is continually carried back, the lengthening of the embryo always taking place between the front end of the primitive streak and the last somite; and during this process the primitive streak undergoes important changes both in itself and in its relation to the embryo. Its anterior thicker part, which is enveloped in the diverging medullary folds, soon becomes distinguished in structure from the part behind this, and placed symmetrically in relation to the axis of the embryo ([fig. 103], a.pr), and at the same time the medullary folds, which at first simply diverge on each side of the primitive streak, bend in again and meet behind so as completely to enclose the front part of the primitive streak. The region of the embryo bird, where the medullary folds diverge, is known as the sinus rhomboidalis, though it has no connection with the similarly named structure in the adult. By the time that ten somites are formed the sinus rhomboidalis is completely established, and the medullary groove has become converted into a tube till close up to the front end of the sinus. In the following stages the closure of the medullary canal extends to the sinus rhomboidalis, and the folding off of the hind end of the embryo from the yolk commences. Coincidently with the last-named changes the sides of the front part of the primitive streak become thickened, and give rise to conspicuous caudal swellings; in which the layers of the embryo are indistinguishably fused. The apparently hinder part of the primitive streak becomes, as more particularly explained in the sequel, folded downwards and forwards on the ventral side.
Fig. 103. Dorsal view of the hardened blastoderm of a Chick with five mesoblastic somites. The medullary folds have met for part of their extent, but have not united.
a.pr. anterior part of the primitive streak; p.pr. posterior part of the primitive streak.
This is a convenient place to notice remarkable appearances which present themselves close to the junction of the neural plate and the primitive streak. These are temporary passages leading from the hinder end of the neural tube into the alimentary canal. They vary somewhat in different species of birds, and it appears that in the same species there may be several openings of the kind, which appear one after the other and then close again. They were first discovered by Gasser (No. [127]). In all cases[64] they lead round the posterior end of the notochord, or through the point where the notochord falls into the primitive streak.
If the primitive streak is, as I believe, formed of the lips of the blastopore, there can be but little doubt that these structures are disappearing, and functionless rudiments of the opening of the blastopore, and they thus lend support to my view as to the nature of the primitive streak. That, in part, they correspond with the neurenteric canal of the Ichthyopsida is clear from the detailed statements below. Till their relations have been more fully worked out it is not possible to give a more definite explanation of them.