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TEXT-BOOK OF EMBRYOLOGY. 



primitive groove, all three germ layers are blended into a solid mass of cells 

 (Fig. 66). On the ground that the primitive groove is the blastopore, the meso- 

 derm here is the peristomal mesoderm, the homologue of the peristomal 

 mesoderm which encircles the blastopore in lower forms (Fig. 37). 



Primitive groove and folds 



Ectoderm 



Mesoderm 



Entoderm 



FIG. 66. Transverse sections of blastoderm of chick (21 hours' incubation). 



a, Section through primitive groove, posterior to Hensen's node. 



b, Section through Hensen's node. 



Hertwig. 



At a somewhat later stage, after the head process appears, sections through 

 the head process also show all three germ layers. Here the ectoderm is a sepa- 

 rate layer; but the entoderm and mesoderm are fused in the medial line; that 



Head process Neural plate 



Ectoderm 



Mesoderm 

 - Entoderm 



Yolk cell 



Archenteron 



FIG. 67. Transverse section of blastoderm of chick (21 hours' incubation). Hertwig. 

 through head process, anterior to Hensen's node. 



Section 



is, in the line of the "primitive intestinal cord." Laterally, the layers are all 

 separate, a cleft existing between the mesoderm and the ectoderm and another 

 between the mesoderm and the entoderm (Fig. 67). Since the mesoderm in 

 the region of the head process is in front of the primitive groove (blastopore) 



