FORMATION AND FATE OF THE PRIMITIVE STREAK. 115 



Externally to this and posteriorly the section cuts another portion 

 of primitive streak (PS' V ), 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 (PC), together with the canal {PSG) 

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

 and laterally and dor sally by the folded-over anterior portions of the 

 lateral lips of the blastopore (PS iv ). 



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 tube has 

 become separated from the skin, and with it the dorsal portion of the 

 primitive streak. 



The primitive streak should, however, still be connected with the 

 skin ventrally by that portion which does not become folded up and 

 nipped off as does the dorsal portion, together with the neural tube. 



Possibly this connection may exist for a very short time, but prac- 

 tically the separation from the skin of the dorsal moiety takes place 

 contemporaneously with the splitting up of the ventral moiety, so that 

 the space between the dorsal moiety of the primitive streak and the 

 skin from which it has separated, and the space between the posterior 

 wall of the rectal spout and post-anal gut on the one hand, and the 

 skin at the same level caused by the splitting up of the primitive 

 streak of that area, becomes confluent at the point in fig. 26 just where 

 the line PS' crosses the space between primitive streak and skin. 



Thus it comes about that the dorsal portion of the primitive streak, 

 which remains " functional " and gives rise to the greater part of the 

 tail, comes to lie entirely within the embryo, 



