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



Fig. 26 represents the stage at which the tail has definitely begun 

 to grow. The dorsal moiety of the primitive streak is seen to be lying 

 within the embryo, and it is by proliferation of its cells that the whole oj 

 the tail is formed with the exception of the skin. In no section after 

 (the stage shortly antecedent to this) have we been able to see a, fusion 

 between the skin and the primitive streak, though the skin lies closely 

 over the primitive streak. 



The skin would seem to grow, not at any one point, but over its 

 whole surface, on account of the pressure caused from within by the 

 growth of the main axis of the body, in response to which the skin 

 must either grow or rupture. 



To a certain extent the skin of the middle line of the ventral surface 

 of the tail is derived from the primitive streak ; that is from the 

 ventral moiety by the splitting up of the latter. This portion is dis- 

 tinguished in fig. 26 by a different mode of shading ; but with the 

 exception of this, which we think extends only a 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 concrescence 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 PS™ 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 {Rana temporaria), by con- 

 crescence 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 



