FORMATION AND FATE OF THE PRIMITIVE STEEAK. 487 



blastopore have been folded up along with the neural folds, 

 and the adjoining areas of epiblast marked by the daggers 

 will shortly meet and fuse, just as will the areas of epiblast 

 adjoining the lateral edges of the neural plate marked by the 

 asterisks. 



Similarly, just as after the meeting and fusing of the 

 epiblast along the lateral edges of the neural plate has taken 

 place, and just as the neural plate — or tube, as it now will be 

 — separates from and lies within the skin, so also will that 

 portion of the primitive streak separate from the skin and 

 come to lie within the embryo. 



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 con- 

 nection 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 por- 

 tion 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 



