94 



TEXT-BOOK OF EMBRYOLOGY. 



reasonable to suppose that it has arisen outside the embryonic disk and united 

 with the intraembryonic mesoderm secondarily. 



While neither the origin of the extraembryonic mesoderm, nor its behavior 

 up to the stages in Peters' and Graf Spec's ova, has been observed in man, it is 

 possible to construct hypothetical diagrams which allow of comparison with 

 what actually occurs in the lower Mammals. The morula, the differentiation 



iotic Cavity 

 Em&fr0j}te Disk 



Ectoderrrj 

 Lrjtoderm 



Visceral __ 

 Mesoderrrj 



Parietal 



Mesoderm 



FIG. 89. Diagrams representing hypothetical stages in the development of the 



human embryo (to follow Fig. 88). 



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. 52. B, Mesoderm (represented by dotted portion) has appeared 

 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 anlage of the extraembryonic 

 body cavity (exoccelom). 



of the superficial layer of cells, the formation of the trophoderm and inner cell 

 mass, and the differentiation of the primary entoderm may be represented 

 hypothetically by the diagrams in Fig. 88. These are quite comparable with 

 the corresponding stages of development in the bat (Fig. 33). In Fig. 89, A, 

 the amniotic cavity formed by a vacuolization of a part of the inner cell mass is 

 shown, and also the entoderm lining the entire yolk cavity. This is also com- 



