50 



Growth in length of the Vertebrate Embryo 



At the same time the deuterogenetic centre has been acting 

 just as it does in an Amphibian; that is to say dorsally it has 

 been adding on new tissue to the previously existing tissue, new 

 ectoderm to the primarily formed ectoderm,, new neural plate to 

 that previously formed and so on, producing growth in length. 



This dorsal region which is homologous to the dorsal lip of 

 the blastopore therefore grows backwards, just as the dorsal lip 

 of the blastopore of the Frog grows back, and in some cases there 

 results a condition very similar to that found in the Frog. But in 

 spite of the similarity in appearance there is an all-important 

 difference. 



The dorsal and dorso-lateral lips in the Frog grow back over 

 the yolk, and an endodermal yolk plug eventually fills the opening 

 of the blastopore and projects out through it for a time. 



ABC 



Fig, 29. A, Amphibian; B, Reptile; C, Bird or Mammal. 

 Small dots, protogenetic tissue ; large dots, yolk ; lines, deuterogenetic tissue. 



In the Reptile, the dorsal lip, and the dorso-lateral lips are 

 growing back over the morphologically coalesced ventro-lateral 

 and ventral lips of the blastopore not over yolk or endoderm at 

 all that is to say, not over protogenetic tissue but over deutero- 

 genetic tissue. 



There is no blastopore in the Reptile, and therefore there can 

 be no yolk plug. 



The canal formed in this way is neurenteric canal as Kupffer 

 correctly named it. It has also been termed mesodermal sac by 

 Hertwig. It is wholly deuterogenetic. 



The accompanying diagram, Fig. 29, should make this clear 

 as seen in surface view. The small dots, large dots and shaded 

 lines represent the protogenetic tissues, yolk and deuterogenetic 

 tissues respectively. 



