GERM LAYERS. 



83 



mesoderm is a derivative of the protentoderm or invaginated cells. Also, as the 

 invagination is a continuous process from the first formation of the crescentic 



Ectoderm 



Neural 

 tube 



Entoderm 



Ccelom 



Fig. 72. — Transverse section of chick embryo (2 days incubation). Photograph. 

 The parietal mesoderm (lying above the ccelom) is not labeled. The two large vessels under 

 the primitive segments are the primitive aortae. Spaces separating germ layers are due to 

 shrinkage. 



groove up through the formation of the "primitive intestinal cord" (see p. 66), 

 one can readily understand how the mesoderm is first formed in the line of the 

 primitive groove and continues to be formed progressively forward as the invagi- 



Area pellucida 

 Area vasculosa 



Head fold 

 Neural groove 



Primitive segment 

 Primitive groove 



Fig, 73 —Dorsal view of duck embryo, with two pairs of primitive segments. Bonnet. 



nation pushes farther and farther forward to form the "primitive intestinal 

 cord." The gastral mesoderm is thus from its beginning continuous with the 

 peristomal mesoderm, the two together forming a single plate of cells. 



