16 



Growth in length of the Vertebrate Embryo 



those cells to which we alluded in the preceding paragraph. But 

 in any case there is undoubtedly some splitting, even if it is only 

 such as is sufficient to make the cavity derived from the segmenta- 

 tion cavity continuous with that derived from the overgrowth of 

 the blastopore lip. 



It might be well to restrict the term archenteron to the proto- 

 genetic part of the gut cavity and to call the deuterogenetic part 

 metenteron ("neo-enteron" of de Lange), Fig. 9. 



Rana. Diagram to show protogenetic and deutero- 

 genetic tissues with a dividing line A, B. 



ec, en protogenetic ectoderm and endoderm ; ec'. en' deuterogenetic ectoderm and 

 endoderm; dl dorsal lip of blastopore ; vl ventral lip of blastopore ; a archen- 

 teron ; m metenteron. 



At first sight a difficulty arises because the cavity in question 

 seems to be roofed in by deuterogenetic, while it is paved by 

 protogenetic tissue. Really however the portion of the floor 

 which corresponds to the deuterogenetic roof is the little piece 

 near the letter v, Fig. 10. The yolk plug is the hypertrophied 

 floor of the protogenetic area or true archenteron, but it is pro- 

 jected out temporarily through the blastopore mouth. At a later 

 stage the yolk plug is actually withdrawn this in itself may 

 perhaps be an invagination process. 



The reason why the roof of the metenteron is so much longer 

 than the floor, is that the dorsal lip by which the roof is formed 

 comes into being so much sooner in the frog than does the ventral 

 lip. 



For a time the whole annular area of deuterogenetic activity 



