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. (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).
Fig. 129—Longitudinal section of the hinder end of a chick. (From Balfour.) sp medullary tube, connected with the terminal gut (pag) by the neurenteric canal (ne), ch chorda, pr neurenteric (or Hensen’s) ganglion, al allantois, ep ectoderm, hy entoderm, so parietal layer, sp visceral layer, an anus-pit, am amnion.