GERM LAYERS. 



93 



Parietal mesoderm Primitive groove 



Visceral mesoderm | i Primitive fold 



Entoderm 

 FIG. 87. Transverse section through primitive groove of rabbit embryo, van Beneden. 



D 



FIG. 88. Diagrams representing hypothetical stages in the development of the human embryo. 



A, Morula; compare with Fig. 33, a. B, Morula with differentiated superficial cells; compare with 

 Fig. 33, b. C, Central cells have become vacuolized to form the yolk cavity, leaving a small 

 group (the inner cell mass) attached to the enveloping layer (trophoderm) ; compare with 

 Fig. 33, d. D, Cells of the inner cell mass which are adjacent to the yolk cavity have become 

 differentiated and have begun to grow around the cavity, forming the entoderm; compare 

 with Fig. 52, a. 



