Fig.129. Longitudinal section of the hinder end of a chick. 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. The complete bilateral symmetry of the vertebrate body is very early indicated in the oval form of the embryonic shield (Fig. 117) by the median primitive streak; in the sandal-form it is even more pronounced (Figs. 131–135). 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 (Fig. 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. Fig.130. Germinal area or germinal disk of the rabbit, with sole-shaped embryonic shield. Fig. 130—Germinal area or germinal disk of the rabbit, with sole-shaped embryonic shield, magnified. The clear circular field (_d_) is the opaque area. The pellucid area (_c_) is lyre-shaped, like the embryonic shield itself (_b_). In its axis is seen the dorsal furrow or medullary furrow (_a_). (From _Bischoff_.