GERM LAYERS. 



85 



archenteron being a flat cavity between the entoderm and the yolk (Figs. 67, 68 

 and 69). The tubular form of the intestine is brought about later in connection 

 with the constriction of the embryo from the yolk sac (p. 140; see also forma- 

 tion of primitive gut, p. 317). 



Mesoderm Formation in Mammals. In Mammals the same difficulties 

 are met with in determining the origin of the mesoderm as in the chick. At the 

 same time, transverse sections through the developing mammalian blastoderm 



Ectoderm 



Mesoderm 



Ba 



Meso- 

 derm 



Mesoderm 



Yolk entoderm Pr.st 



FIG. 75. Transverse sections of embryonic disk of dog. Bonnet. 

 Sections of disk shown in Fig. 53. Letters and numbers at right (Sj-S 4 ) indicate plane of sections 

 in Fig. 53. P.gr., Primitive groove; Pr.int.co., primitive intestinal cord; Pr.st., all three 

 germ layers fused in primitive streak. 



at different stages show conditions which bear much resemblance to those in the 

 chick, and lead toward the conclusion that the processes in the two cases are 

 much alike. 



Referring back to gastrulation, it will be remembered that on surface view 

 the germ disks of the chick and of the dog were very similar (compare Fig. 46 

 with Fig. 53, and Fig. 50 with Fig. 54, a). After the formation of the primitive 

 streak in the dog, sections through this region show the mesoderm interposed 

 between the ectoderm and entoderm (here yolk entoderm) and all three germ 



