490 AETHUR ROBINSON AND RICHARD ASSHfiTON. 



short way, no part of the skin of the tail is derived from the 

 primitive streak. 



The neurenteric canal is undoubtedly to be regarded as the 

 most dorsal part of the blastopore ; and, although it remains 

 open for a short time after the commencement of the tail, it 

 gradually closes, as we have shown in the semi-diagrammatic 

 figure, fig. 26, NU. This closing may, perhaps, be not incorrectly 

 spoken of as the continuation and completion of the concres- 

 cence from behind forwards which caused the closure of the 

 ventral portion of the original blastopore. 



As far as we have been able to observe, the post-anal gut 

 only exists during the persistence of the neurenteric canal. 



In fig. 26 the part of primitive streak P/S'^ has been drawn 

 too large proportionately to the part PS". 



Conclusions as regards the Primitive Streak; its 

 Origin and Fate. 



A structure exactly comparable to the primitive streak in 

 the chick, median and grooved, is formed in the frog (Ran a 

 temporaria) by concrescence of the lips of the blastopore 

 from behind forwards. 



The anus perforates the posterior or ventral end of the 

 primitive streak, being a deepening of the primitive groove. 

 It may, therefore, be regarded as the reopening of the most 

 ventral part of the blastopore. 



The portion of the primitive streak with which the dorsal 

 wall of the archenteron, notochord, and floor of the neural 

 tube are continuous is the most anterior limit of the primitive 

 streak, and is the dorsal or most anterior lip of the blastopore. 



The neurenteric canal, which is bounded anteriorly by the 

 dorsal lip of the blastopore, is therefore the most anterior por- 

 tion of the blastopore. 



The ventral moiety of the primitive streak shortly after the 

 perforation of the anus ceases to exist, or, as we have preferred 

 to term it, ceases to be " functional,'^ and splits up. 



The dorsal moiety of the primitive streak becomes folded 

 upon itself like, and along with, the neural plate, and becomes 



