GERM LAYERS. 



of entodermal cells called the notochord. These may be seen in cross 

 section in Fig. 19, G and H. In G, on either side of the medullary groove 



Coe. 



Neph 



FIG. 19. DIAGRAMS SHOWING THE DEVELOPMENT OP THE GERM LAYERS. (A to F, after van Beneden's 



figures of the rabbit.) 



A, Two-celled stage; B, four-celled stage; and C, morula stage of the segmenting ovum, all being surface 

 views. D to I represent sections described in the text. The inner cell mass and entoderm are 

 heavily shaded; the outer layer and ectoderm are light; and the mesoderm is represented by dashes. 

 Coe., coelom or body cavity. Int., intestinal cavity. Neph., nephrotome. Seg., mesodermic 

 segment. 



and notochord a third layer of cells appears between the ectoderm and 

 entoderm, and it gradually encircles the vesicle as did the entoderm. It 

 is the mesoderm, which has an obscure origin near the primitive streak. 



