Comparison of Rabbit and Frog 



35 



(i) The general radial character of the protogenetic area, 



(ii) After a certain moment the primitive streak becomes 

 contracted shortened and thickened, 



(iii) An additional part of the embryo has been intercalated 

 between the primitive streak and the radial protogenetic area. 



This intercalated region is approximately all that part of the 

 embryo which includes the protovertebrae. Now we find that in 

 the Amphibian (Frog) it is just that part which is formed from the 

 lips of the blastopore. 



If we can assume that this intercalated region has been added 

 by, and at the expense of, the primitive streak, we arrive at the 



Fig. 18. A, Frog; B, Rabbit. 



Corresponding deuterogenetic proliferating areas ; vl ventral ; 

 dl dorsal lip of blastopore. 



conclusion that physiologically the primitive streak is identical 

 with the blastopore lip region of the Anamnia. 



To prove this by experiment is probably impossible in 

 Mammals, but in Birds the supposition has been put to a practical 

 test. 



If we can assume that the primitive streak of the Bird is the 

 same as that of the Rabbit, then we can say that experiment 

 proves that the primitive streak is converted into the meta- 

 merically segmented part of the animal, but that the anterior 

 part of the animal is developed quite independently of the 

 primitive streak. 



32 



