While these important processes are taking place in the axial part of the dorsal shield, its external form also is changing. The oval form (Figure 1.117) becomes like the sole of a shoe or sandal, lyre-shaped or finger-biscuit shaped (Figure 1.130). 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 (Figure 1.131 a). The completion of the circle in the area marks the limit of the formation of blood-vessels in the mesoderm. (FIGURE 1.129. 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.) 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 coelom-folds has begun (Figure 1.132). 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 (Figure 1.133). The complete bilateral symmetry of the vertebrate body is very early indicated in the oval form of the embryonic shield (Figure 1.117) by the median primitive streak; in the sandal-form it is even more pronounced (Figures 1.131 to 1.135). In the lateral parts of the embryonic shield a darker central and a lighter peripheral zone become more obvious; the former is called the stem-zone (Figure 1.134 stz), and the latter the parietal zone (pz); from the first we get the dorsal and from the second the ventral half of the body-wall. The stem-zone of the amniote embryo would be called more appropriately the dorsal zone or dorsal shield; from it develops the whole of the dorsal half of the later body (or permanent body)--that is to say, the dorsal body (episoma). Again, it would be better to call the "parietal zone" the ventral zone or ventral shield; from it develop the ventral "lateral plates," which afterwards separate from the embryonic vesicle and form the ventral body (hyposoma)--that is to say, the ventral half of the permanent body, together with the body-cavity and the gastric canal that it encloses.