In order to get a clear idea of this, we must understand carefully the relation of the embryonic shield to the germinative area and the embryonic vesicle. This is done best by a comparison of the five stages which are shown in longitudinal section in Figs. 138–142. The embryonic shield (c), which at first projects very slightly over the surface of the germinative area, soon begins to rise higher above it, and to separate from the embryonic vesicle. At this point the embryonic shield, looked at from the dorsal surface, shows still the original simple sandal-shape (Figs. 133–135). We do not yet see any trace of articulation into head, neck, trunk, etc., or limbs. But the embryonic shield has increased greatly in thickness, especially in the anterior part. It now has the appearance of a thick, oval swelling, strongly curved over the surface of the germinative area. It begins to sever completely from the embryonic vesicle, with which it is connected at the ventral surface. As this severance proceeds, the back bends more and more; in proportion as the embryo grows the embryonic vesicle decreases, and at last it merely hangs as a small vesicle from the belly of the embryo (Fig. 142 ds). In consequence of the growth-movements which cause this severance, a groove-shaped depression is formed at the surface of the vesicle, the limiting furrow, which surrounds the vesicle in the shape of a pit, and a circular mound or dam (Fig. 139 ks) is formed at the outside of this pit by the elevation of the contiguous parts of the germinal vesicle.
Fig. 136—Transverse section of the embryonic disk of a chick at the end of the first day of incubation, magnified. The edges of the medullary plate (m), the medullary swellings (w), which separate the medullary from the horn-plate (h), are bending towards each other. At each side of the chorda (ch) the primitive segment plates (u) have separated from the lateral plates (sp). A gut-gland layer. (From Remak.)
In order to understand clearly this important process, we may compare the embryo to a fortress with its surrounding rampart and trench. The ditch consists of the outer part of the germinative area, and comes to an end at the point where the area passes into the vesicle. The important fold of the middle germinal layer that brings about the formation of the body-cavity spreads beyond the borders of the embryo over the whole germinative area. At first this middle layer reaches as far as the germinative area; the whole of the rest of the embryonic vesicle consists in the beginning only of the two original limiting layers, the outer and inner germinal layers. Hence, as far as the germinative area extends the germinal layer splits into the two plates we have already recognised in it, the outer skin-fibre layer and the inner gut-fibre layer. These two plates diverge considerably, a clear fluid gathering between them (Fig. 140 am). The inner plate, the gut-fibre layer, remains on the inner layer of the embryonic vesicle (on the gut-gland layer). The outer plate, the skin-fibre layer, lies close on the outer layer of the germinative area, or the skin-sense layer, and separates together with this from the embryonic vesicle. From these two united outer plates is formed a continuous membrane. This is the circular mound that rises higher and higher round the whole embryo, and at last joins above it (Figs. 139–142 am). To return to our illustration of the fortress, we must imagine the circular rampart to be extraordinarily high and towering far above the fortress. Its edges bend over like the combs of an overhanging wall of rock that would enclose the fortress; they form a deep hollow, and at last join together above. In the end the fortress lies entirely within the hollow that has been formed by the growth of the edges of this large rampart.
Fig. 137—Three diagrammatic transverse sections of the embryonic disk of the higher vertebrate, to show the origin of the tubular organs from the bending germinal layers. In Fig. A the medullary tube (n) and the alimentary canal (a) are still open grooves. In Fig. B the medullary tube (n) and the dorsal wall are closed, but the alimentary canal (a) and the ventral wall are open; the prorenal ducts (u) are cut off from the horn-plate (h) and internally connected with segmental prorenal canals. In Fig. C both the medullary tube and the dorsal wall above and the alimentary canal and ventral wall below are closed. All the open grooves have become closed tubes; the primitive kidneys are directed inwards. The letters have the same meaning in all three figures: h skin-sense layer, n medullary tube, u prorenal ducts, x axial rod, s primitive-vertebra, r dorsal wall, b ventral wall, c body-cavity or cœloma, f gut-fibre layer, t primitive artery (aorta), v primitive vein (subintestinal vein), d gut-fibre layer, a alimentary canal.
As the two outer layers of the germinative area thus rise in a fold about the embryo, and join above it, they come at last to form a spacious sac-like membrane about it. This envelope takes the name of the germinative membrane, or water-membrane, or amnion (Fig. 142 am). The embryo floats in a watery fluid, which fills the space between the embryo and the amnion, and is called the amniotic fluid (Figs. 141, 142 ah). We will deal with this remarkable formation and with the allantois later on (Chapter XV). In front of the allantois the yelk-sac or umbilical vesicle (ds), the remainder of the original embryonic vesicle, starts from the open belly of the embryo (Fig. 138 kh). In more advanced embryos, in which the gastric wall and the ventral wall are nearly closed, it hangs out of the navel-opening in the shape of a small vesicle with a stalk (Figs. 141, 142 ds). The more the embryo grows, the smaller becomes the vitelline (yelk) sac. At first the embryo looks like a small appendage of the large embryonic vesicle. Afterwards it is the yelk-sac, or the remainder of the embryonic vesicle, that seems a small pouch-like appendage of the embryo (Fig. 142 ds). It ceases to have any significance in the end. The very wide opening, through which the gastric cavity at first communicates with the umbilical vesicle, becomes narrower and narrower, and at last disappears altogether. The navel, the small pit-like depression that we find in the developed man in the middle of the abdominal wall, is the spot at which the remainder of the embryonic vesicle (the umbilical vesicle) originally entered into the ventral cavity, and joined on to the growing gut.
The origin of the navel coincides with the complete closing of the external ventral wall. In the amniotes the ventral wall originates in the same way as the dorsal wall. Both are formed substantially from the skin-fibre layer, and externally covered with the horn-plate, the border section of the skin-sense layer. Both come into existence by the conversion of the four flat germinal layers of the embryonic shield into a double tube by folding from opposite directions; above, at the back, we have the vertebral canal which encloses the medullary tube, and below, at the belly, the wall of the body-cavity which contains the alimentary canal (Fig. 137).