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



The posterior or ventral portion of the primitive streak does not 

 become folded in this way, for by the time the folding of the dorsal 

 portion has been completed and the separation from the skin has taken 

 place, the lower or ventral portion has, as we have described above, 

 split up and ceased to exist as a " functional " primitive streak. 



By this means the primitive streak loses all direct connection with 

 the surface epiblast, though the connection of the primitive streak 

 with the surface epiblast may be said to be in reality retained by the 

 direct continuity of the cells of the primitive streak with the interior 

 lining (i.e. epidermic epiblast) of the neural tube. 



The semi-diagrammatic sagittal sections, figs. 24, 25, and 26, will 

 aid in rendering clearer the above statements. 



Fig. 24 is an early stage, only a little older than the stage 

 represented by fig. 18. 



The " primitive streak," or area of fused layer which is cut through 

 in these three sections, is shown by a diagonal shading. 



In fig. 24 the mass of primitive streak is seen to be much greater 

 ventrally than dorsally ; the extent of closure of the original blastopore 

 is expressed by the distance between x and the inner edge of the lip 

 bounding ventrally the still open portion of the blastopore. 



In fig. 25 the neural plate has become folded upon itself and fused, 

 but the neural tube so formed has not yet separated from the skin, — 

 it is, in fact, on the point of so doing ; it therefore 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 (PJS") continuous with 

 the dorsal wall of the archenteron (HY), the notochord (CH), and 

 floor of the neural tube, which is the actual dorsal lip of the blasto- 

 pore, 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), notochord (GH), floor of neural tube (NP) — in 

 fig. 24. 



