142 



TEXT-BOOK OF EMBRYOLOGY. 



the ventral body wall. This line of fusion is significant in its relation to certain 

 malformations (Chap. XIX). 



Preceding the processes which establish the cylindrical form of the body, 

 there are changes in the relation of the amnion to the chorion. Primarily, the 

 entire dome-like roof of the amniotic cavity is attached to the chorion (Fig. 90, A). 

 In further development, however, the extraembryonic mesoderm between the 

 trophoderm of the chorion and the ectoderm of the amnion splits farther back 

 over the embryo, leaving the latter attached at its caudal end to the chorion by a 

 mass of mesoderm the so-called belly stalk (Figs. 90, B, and 85). 



Following the above mentioned changes in the amnion, chorion, yolk sac 

 and embryonic disk, the amnion continues to enlarge and thus draws the belly 



Cephalic 

 flexure 



Branchial arches 



Branchial grooves 



Heart 



Dorsal flexure 



Yolk sac 



FIG. i2i. Human embryo 2.15 mm. long. His. 



stalk under the embryonic body and brings it closer to the yolk sac. Finally, as 

 the yolk stalk becomes longer and more slender, the belly stalk and yolk stalk 

 unite and become completely surrounded by the amnion. There is thus formed 

 a cord-like structure-the umbilical cord which is attached to the ventral side 

 of the body (Figs. 90, D, and 100; see also p. 132). 



The changes which occur in the simple cylindrical body, after it is once 



ormed, consist of the differentiation of the head, neck and body regions and the 



lopment of the extremities. Even in Eternod's embryo (Fig. 119) the 



cephalic end has become proportionately larger than the rest of the body and 



somewhat beyond the yolk sac. This marks the beginning of the 



