FORMATION AND FATE OF THE PRIMITIVE STREAK. 113 



dotted NP. The space BR between the two small concentric circles 

 represents the position of the fused layers — that is to say, the lips of 

 the blastopore at the time of the first formation of the blastopore. 



The shaded space BR' represents the position of the same fused 

 layers in fig. 18 at the time of the complete formation of the neural 

 plate, but before it has commenced to fold up; in fig. 19 after the 

 neural plate has become completely folded, and its outer lateral edges are 

 just meeting. 



BU is that portion of the blastopore which remains open longest. 



The two asterisks in fig. 18 mark the two lines of epiblast im- 

 mediately adjoining the lateral edges of the neural plate. 



When the neural plate has become folded, and when its lateral edges 

 have met and fused, and separated from the adjoining epiblast, the lines 

 of epiblast indicated by the asterisk will have fused and made good the 

 gap which would otherwise be caused in the skin by the separation 

 from it of the neural plate. 



In fig. 19 the folds are represented as having nearly met, so that 

 the outer surface (dotted) of the neural plate is now no longer seen, 

 except a narrow strip through the now rapidly approaching lateral edges 

 of the neural plate. 



The daggers similarly mark two spots in the epiblast adjoining the 

 fused layers at the outer edges of the lateral lips of the still open 

 portion of the blastopore. 



In fig. 18 the relation of the neural plate to the dorsal lip of the 

 blastopore — that is to say, to the dorsal end of the primitive streak — is 

 clearly represented. 



In fig. 19 the folding up of the neural plate is nearly complete, and 

 with it the dorsal part of the primitive streak — that is to say, the 

 dorsal portions of the lateral lips of the 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 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. 

 H 



