112 ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



The difference in character of cell between the posterior wall and 

 anterior wall of the rectal spout is very marked, as 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 different. 



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

 to our definition above, this portion of the primitive 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. 



Eeference 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 whatever 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 anterior 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 ; and the neural canal, if it opened anywhere, 

 would open to the exterior dorsal to the blastopore, and not into the 

 blastoporic canal, as is well known to be the actual case. 



This we have tried to make clear by the diagrams, fig. 18 and fig. 19. 



In these diagrams the external surface of the neural plate has been 



