488 ARTHUE EOBINSON AND EICHARD ASSHETON. 



from the skin, — it is^ in fact^ on the point of so doing; it there- 

 fore represents a stage rather later than that of fig. 19. 



Ventrally, the primitive streak is in the middle line divided into 

 a post- anal (PS') and pre-anal portion {PS"') by the deepening 

 of the primitive groove at one spot (A), which shortly after 

 completely perforates and forms the permanent anus. As we 

 have stated above^ we regard this as practically a reopening of 

 the ventral part of the original blastopore. This is also shown 

 diagrammatically in fig. 19, A. 



Dorsally, we get a portion of primitive streak (PS'') continu- 

 ous with the dorsal wall of the archenteron [HY), the noto- 

 chord [CH), and floor of the neural tube, which is the actual 

 dorsal lip of the blastopore, and is exactly the same as that 

 portion of primitive streak which bears the same relations to 

 the same structures — dorsal wall of archenteron {HY), noto- 

 chord {CH), floor of neural tube (iVP)— in fig. 24. 



Externally to this and posteriorly the section cuts another 

 portion of primitive streak {PS'''), which is the dorsal part of 

 the lateral lips of blastopore which have been folded up along 

 with the neural folds, as we have already described, and as we 

 have endeavoured to represent diagrammatically in fig. 19. 



By this means a canal is formed leading from the archenteron 

 to the neural tube, known as the neurenteric canal, which 

 is therefore the anterior portion of the blastopore 

 (5C),together with the canal (P/SC) bounded ventrally 

 by the anterior portion of the primitive streak (PS"), 

 and laterally and dorsally by the folded-over anterior 

 portions of the lateral lips of the blastopore {PS'"). 



At this period there is still a direct passage to the exterior 

 from the archenteron, as well as into the neural tube ; but as 

 the folding up of the dorsal portion of the primitive streak 

 progresses, combined probably with the continued concrescence 

 of the more ventral portion of the lateral lips of the blastopore, 

 this external opening — that is to say, that portion of the original 

 blastopore which apparently remains open for the longest 

 period — is entirely and finally closed from the exterior. 



By the time this has occurred, or shortly after, the neural 



