FORMATION AND PATE OF THE PEIMITIVE STREAK. 485 



shown especially well in fig. 20. This feature is not so marked 

 in stained specimens, where the degree of pigmentation does 

 not show up so well. 



This description so far agrees with Goette (15), and 

 Schwarz (47), who follows Goette in this respect, except that 

 we include the ventral lip of the anus within the primitive 

 streak, which apparently these authors do not, though for 

 what reasons it is not easy to understand. 



Fate of the Dorsal Moiety of the Primitive 

 Streak, and Development of the Tail. 



In the dorsal moiety the fate of the primitive streak is 

 diflFerent. 



Instead of splitting up, it remains as a proliferating area ; 

 or according to our definition above, this portion of the primi- 

 tive streak remains for a longer time " functional." 



The relation of the neural folds to the blastopore, that is 

 to say to the primitive streak, must be necessarily considered 

 in connection with the fate of the dorsal moiety of the 

 primitive streak. 



Reference to fig. 14, which passes through the rapidly 

 closing blastopore {BL'), conclusively proves that there is here 

 no trace of neural folds. The epiblast shows no signs what- 

 ever of any thickening, the mass of cells being merely the 

 fused layers — that is to say, the undifferentiated cells at the 

 lips of the blastopore — of typical primitive streak. 



There is no trace of neural folds — that is, of thickened 

 epiblast separate from mesoblast — until several sections an- 

 terior to the still open portion of the blastopore. That is to 

 say, that which from a surface examination appears to be 

 the lower end of the neural folds, is really the dorsal portion 

 of the lateral lips of the blastopore folded up along with the 

 true neural folds. 



If this were not so, there would be a sudden break at the 

 posterior end of the neural folds j and the neural canal, if it 

 opened anywhere, would open to the exterior dorsal to the bias- 



