138 TEXT-BOOK OF EMBRYOLOGY. 



(Fig. 89; compare with Fig. 52). The further development of the latter has 

 been described on page 116. 



At this stage the sphere contains two cavities, the larger yolk cavity and the 

 smaller amniotic cavity, separated by a double layer of cells, the ectoderm and 

 entoderm, which constitute the embryonic disk. The greater part of the wall of 

 the sphere is composed of two layers; the portion forming the wall of the larger 

 yolk cavity being composed of trophoderm and entoderm, the portion forming 

 the wall of the smaller amniotic cavity being composed of trophoderm alone 

 (Fig. 89). The entire structure is spoken of as the blastodermic vesicle. 



FIG. 117. Human embryo of two months (twenty-six millimeters). Photograph. 

 The embryo lies within the chorion (open on one side), to which it is attached at the right of the 

 figure by the umbilical cord; around the point of attachment the chorionic villi can be seen. 

 The amnion has been opened and turned back. 



The formation of the mesoderm has been discussed elsewhere (Chap. VI, 

 p. 85). At this point it is sufficient to say that it appears in the wall of the 

 yolk cavity as a third layer between the trophoderm and entoderm, and, in the 

 embryonic disk, between the ectoderm and entoderm. Thus the blastodermic 

 vesicle possesses all three germ layers (Fig. 89). 



In the further course of development the mesoderm splits into two layers, 

 an outer or parietal and an inner or visceral. Between the layers a cleft ap- 

 pears, which is completely bounded by mesoderm, on the outer side by the 

 parietal, on the inner side by the visceral. The parietal and visceral layers 

 are in apposition to the trophoderm and entoderm respectively. The two 

 layers of mesoderm soon become widely separated owing to rapid growth of 

 the parietal layer and the trophoderm. The parietal layer of mesoderm and 



