DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 109 



Disregarding now the chorion and exoccelom, which are no longer in- 

 volved in the form of the embryonic body, certain advances in development 

 are seen in an embryo described by von Spee. In Fig. 77 a sagittal section 

 shows the large yolk sac separated from the amniotic cavity by the embryonic 

 disk. The anterior margin of the disk is bent ventrally by a fold of the germ 

 layers. Figure 76 shows a dorsal surface view of the embryo; the amnion 

 has been cut away. The embryonic disk is considerably elongated cephalo- 

 caudally. A gutter or groove surrounds the disk, and if compared with the 

 sagittal section which has the fold at the cephalic end it can readily be seen 

 that this groove is an early step in the constriction or pinching off of the 

 disk from the yolk sac. The margins of the disc are being bent ventrally 

 and tucked beneath the central portion; and since the disk is elongated 

 the folding process will result in a cylindrical body form. Even now the 

 impression is obtained that the yolk sac is suspended from the ventral side 

 of the embryo by a narrower structure, the early yolk stalk. The dorsal 

 surface of the disk is indented by the neural groove which extends nearly 

 the whole length of the developing body. 



Somewhat more advanced than the von Spee embryo is one described by 

 Eternod (Fig. 83). Eternod's embryo is 2.11 mm. in length and possesess 

 eight primitive segments. The figure shows the amnion cut away on the 

 dorsal side and the yolk sac on the ventral side. The body is more markedly 

 cylindrical than the preceding stage, and more elongated. The constriction 

 between the embryo and the yolk sac is well marked, and the narrower yolk 

 stalk can be better appreciated. At the caudal end the belly stalk forms the 

 attachment to the chorion. The neural folds are partly fused to form the 

 neural tube. The cephalic end of the neural plate is notably larger, a 

 character which already indicates the beginning of the head. One might 

 say that the yolk stalk is becoming smaller; but as a matter of fact the 

 diminution is more apparent than real. The apparent diminution is caused 

 by the relatively more rapid increase in size of the embryonic body and yolk 

 sac. At this point it should be mentioned that the bending and tucking 

 under the body of the lateral body walls naturally results in the contact 

 and eventual fusion of the two sides in the mid- ventral line. In this manner 

 the ventral body wall is formed. The line of fusion is significant in its rela- 

 tion to certain malformation : For instance, the fusion is sometimes defective 

 or incomplete, allowing some of the viscera to protrude. (See Chap. XX on 

 "Teratogenesis.") If the fusion is normal the ventral body wall is complete 

 and closed except at the attachment of the umbilical cord through which 

 pass the blood vessels that carry nutriment to the embryo and waste products 

 away from it. 



The changes that occur in the simple cylindrical body after the ventral 



