With the formation of the internal navel and the closing of the alimentary canal is connected the formation of two cavities, which we call the capital and the pelvic sections of the visceral cavity. As the embryonic shield lies flat on the wall of the embryonic vesicle at first, and only gradually separates from it, its fore and hind ends are independent in the beginning; on the other hand, the middle part of the ventral surface is connected with the yelk-sac by means of the vitelline or umbilical duct (Fig. 147 _m_). This leads to a notable curving of the dorsal surface; the head-end bends downwards towards the breast and the tail-end towards the belly. We see this very clearly in the excellent old diagrammatic illustration given by Baer (Fig. 147), a median longitudinal section of the embryo of the chick, in which the dorsal body or episoma is deeply shaded. The embryo seems to be trying to roll up, like a hedgehog protecting itself from its pursuers. This pronounced curve of the back is due to the more rapid growth of the convex dorsal surface, and is directly connected with the severance of the embryo from the yelk-sac. The further bending of the embryo leads to the formation of the “head-cavity” of the gut (Fig. 148 above _D_) and a similar one at the tail, known as its “pelvic cavity.”
Fig.147. Median longitudinal section of the embryo of a chick (fifth day of incubation), seen from the right side (head to the right, tail to the left). Fig. 147—Median longitudinal section of the embryo of a chick (fifth day of incubation), seen from the right side (head to the right, tail to the left). Dorsal body dark, with convex outline. _d_ gut, _o_ mouth, _a_ anus, _l_ lungs, _h_ liver, _g_ mesentery, _v_ auricle of the heart, _k_ ventricle of the heart, _b_ arch of the arteries, _t_ aorta, _c_ yelk-sac, _m_ vitelline (yelk) duct, _u_ allantois, _r_ pedicle (stalk) of the allantois, _n_ amnion, _w_ amniotic cavity, _s_ serous membrane. (From _Baer._)
As a result of these processes the embryo attains a shape that may be compared to a wooden shoe, or, better still, to an overturned canoe. Imagine a canoe or boat with both ends rounded and a small covering before and behind; if this canoe is turned upside down, so that the curved keel is uppermost, we have a fair picture of the canoe-shaped embryo (Fig. 147). The upturned convex keel corresponds to the middle line of the back; the small chamber underneath the fore-deck represents the capital cavity, and the small chamber under the rear-deck the pelvic chamber of the gut (cf. Fig. 140).