In the amniotes this original curved form of the neurenteric canal cannot be found at first, because here the primitive mouth travels completely over to the dorsal surface of the gastrula, and is converted into the longitudinal furrow we call the primitive groove. Hence the primitive groove (Fig. 128 _pr_), examined from above, appears to be the straight continuation of the fore-lying and younger medullary furrow (_me_). The divergent hind legs of the latter embrace the anterior end of the former. Afterwards we have the complete closing of the primitive mouth, the dorsal swellings joining to form the medullary tube and growing over it. The neurenteric canal then leads directly, in the shape of a narrow arch-shaped tube (Fig. 129 _ne_), from the medullary tube (_sp_) to the gastric tube (_pag_). Directly in front of it is the latter end of the chorda (_cli_).
Fig.126. Longitudinal section of a frog-embryo. Fig. 126—Longitudinal section of a frog-embryo. (From _Goette._) _m_ mouth, _l_ liver, _an_ anus, _ne_ canalis neurentericus, _mc_ medullary-tube, _pn_ pineal body (epiphysis), _ch_ chorda.
While these important processes are taking place in the axial part of the dorsal shield, its external form also is changing. The oval form (Fig. 117) becomes like the sole of a shoe or sandal, lyre-shaped or finger-biscuit shaped (Fig. 130). The middle third does not grow in width as quickly as the posterior, and still less than the anterior third; thus the shape of the permanent body becomes somewhat narrow at the waist. At the same time, the oval form of the germinative area returns to a circular shape, and the inner pellucid area separates more clearly from the opaque outer area (Fig. 131 _a_). The completion of the circle in the area marks the limit of the formation of blood-vessels in the mesoderm.
Figs. 127 and 128. Dorsal shield of the chick. Figs. 127 and 128—Dorsal shield of the chick. (From _Balfour._) The medullary furrow (_me_), which is not yet visible in Fig. 130, encloses with its hinder end the fore end of the primitive groove (_pr_) in Fig. 131.)
The characteristic sandal-shape of the dorsal shield, which is determined by the narrowness of the middle part, and which is compared to a violin, lyre, or shoe-sole, persists for a long time in all the amniotes. All mammals, birds, and reptiles have substantially the same construction at this stage, and even for a longer or shorter period after the division of the primitive segments into the cœlom-folds has begun (Fig. 132). The human embryonic shield assumes the sandal-form in the second week of development; towards the end of the week our sole-shaped embryo has a length of about one-twelfth of an inch (Fig. 133).