g2 TEXT-BOOK OF EMBRYOLOGY. 



groove and represent the first mesodermal cells (Fig. 69). It is reasonable to 

 assign the origin of these cells to the cells which have been invaginated along the 

 line of the primitive groove (blastopore). These invaginated cells constitute 

 the protentoderm, hence the mesodermal cells may be considered as-derivatives 

 of the protentoderm. 



As proliferation continues, the mesodermal cells spread out between the 

 ectoderm and entoderm (which is here yolk entoderm) (Fig. 70). Finally, the 



Ectoderm p.gr. 



Mesoderm 



Ectoderm 



Entoderm 



Yolk 



FIG. 70. Transverse section of blastoderm of chick (slightly older than that shown in Fig. 69) 



Hertwig. 

 Section taken through primitive groove (p.gr.} and streak. 



mesoderm fuses with the yolk entoderm, so that all three germ layers are fused 

 beneath the primitive groove (Fig. 66). The fusion between the mesoderm and 

 yolk entoderm in this region is a secondary matter. 



That the peristomal mesoderm is a derivative of the invaginated cells is 

 even more clearly demonstrated in Fig. 71, in which the two lips of the blasto- 

 pore have not yet fused. 



Primitive fold Primitive groove 



FIG. 71. Transverse section through primitive streak and primitive groove of Diomedea. 



Schauinsland. 



In front of the primitive groove, that is, in the region of the head process, the 

 gastral mesoderm is at first seen to be continuous with the "primitive intestinal 

 cord" (Fig. 67); later it becomes separated on each side from the "primitive 

 intestinal cord" (now the notochord). While the actual process has not been 

 observed, it is reasonable to assume that the mesoderm is here also a derivative 

 of the "primitive intestinal cord," and since the latter is produced by the in- 

 vagination (gastrulation, see p. 66) and consists of protentoderm, the gastral 



