486 AETHUE EOBINSON AND RICHAED ASSHETON. 



topore, and not into the blastoporic canal, as is well known to 

 be the actual case. 



This we have tried to make clear by the diagrams; fig. 18 

 and fig. 19. 



In these diagrams the external surface of the neural plate 

 has been 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. 



BL' is that portion of the blastopore which remains open 

 longest. 



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

 immediately 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 Hues 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 



