18 Growth in length of the Vertebrate Embryo 



released from the encumbrance and discomforts of a digestive 

 system has led to the wonderful success achieved by the vertebrate 

 phylum. 



It is necessary to realize that the metenteron has a wide roof 

 and narrow floor, because otherwise it is difficult to interpret 

 sections taken vertically and transversely through the egg, e.g. 

 along the line A B, Figs. 9 and 13. 



, mes 



A 



Fig. 10 after de Lange, 1912. Diagram of embryo of Megalobatrachus. 

 an anus; A archenteron; M metenteron; ec protogenetic ectoderm; en proto- 

 genetic endoderm ; dl dorsal lip ; v ventral lip of blastopore ; mes protogenetic 

 mesoderm; mes' deuterogenetic mesoderm; nc neurentenc canal; ec' deutero- 

 genetic ectoderm ; en' deuterogenetic endoderm. 



The deuterogenetic tissues have in the Frog a more compact 

 and less yolky character than the protogenetic tissues. The two 

 regions of course merge one with the other along the lateral walls. 



Thus we get the appearance of a special dorsal plate forming 

 the nerve plate and notochord, Fig. 10. It was this appearance 

 which caused Lwoff in 1894 to describe what he termed the 

 ectoblasto-genetic plate as a turning in of the ectoderm to form 

 the notochord and the roof of the gut cavity. 



