104 



TEXT-BOOK OF EMBRYOLOGY. 



A longitudinal section in the medial sagittal plane shows the embryonic 

 disk separating the yolk cavity from the amniotic cavity (Fig. 77). The 

 mesoderm is an extensive layer investing both amnion and yolk sac and 

 forming a strong band which attaches the embryonic body to the outer wall 

 of the vesicle (now the chorion). A cross section through the primitive 

 streak shows a striking resemblance to a corresponding section of the em- 

 bryonic disk of a rabbit. (Compare Figs. 78 and 79.) The three germ layers 

 are fused in the streak, and the mesoderm extends laterally on both sides 

 between the other two layers. 



Parietal_ 

 Mesoder 



FIG. 81. Diagrams representing hypothetical stages in the development of the 



human embryo (to follow Fig. 80). 



A, Entoderm surrounds the yolk cavity; part of the cells of the inner cell mass have become 

 vacuolated, thus forming the amniotic cavity, while the remainder constitute the embryonic 

 ectoderm; compare with Fig. 59. B, Mesoderm (represented by dotted portion) has ap- 

 peared between the entoderm and trophoderm, between the entoderm and ectoderm of the 

 embryonic disk, and in the roof of the amnion. C, The mesoderm around the yolk cavity 

 has split into a parietal and a visceral layer, the cleft between being the rudiment of the ex- 

 traembryonic body cavity (exoccelom). 



In further development the behavior of the germ layers during the forma- 

 tion of the neural tube, the origin of the mesodermal somites, the appearance 

 of the ccelom in the lateral portion of the mesoderm, and the formation of 



