342 GENERAL EMBRYOLOGY 



begins where the processes of imagination and involution leave 

 off, and it is important to recognize that the didermic character 

 of the gastrula of the Amphibian results partly from all three 

 of these processes. The chief result of involution is the forma- 

 tion of the rudiments of the notochord and gastral mesoderm, 

 as in Amphioxus. Figure 155 shows how the blastocoel is 

 finally obliterated by the invaginated and involuted regions. 

 The germ ring finally completes its growth over the yolk cells 

 or endodermal floor of the archenteron, and closes together much 

 as in Amphioxus. 



The formation of the mesoderm offers some points of differ- 

 ence when compared with Amphioxus. The peristomial meso- 

 derm forms typically in the margin of the blastopore, out of 

 the undifferentiated cell mass of the germ ring. Sometimes, in 

 the region just within the blast opore dorsally, traces of entero- 

 ccelic outgrowths can be seen (Fig. 156), but most of the gastral 



FIG. 156. Part of a section through the body of an embryo of the frog, Rana 

 fusca, showing traces of enteroccel formation. After O. Hertwig. a, archen- 

 teron; c, enterocoels; ec, ectoderm; en, endoderm; m, mesoderm; n, notochord; 

 p, neural plate; y, yolk cells. 



mesoderm is formed either from involuted cells derived from 

 the germ ring, or later from the surface of the endoderm by a 

 process of delamination or splitting off of the superficial cells 

 lying next the ectoderm; these come off first as a solid sheet, 

 which much later itself splits into two layers leaving a coslomic 

 cavity between them. 



Thus while invagination occurs to a slight extent, gastrula- 

 tion in these forms results largely from the processes of involu- 



