FATE OF BLASTOPORE, ETC.. IN CHKLONlA. 75 



the embryonic shield, they embrace tlie yolk-plng- Ijetween their 

 posterior ends ns in a vice, and compress it laterally, thus lifting it 

 up to the level of their dorsal surface (Figs. 2, 1), 17, and 18), The 

 yolk-plug meanwhile keeps its o^vn independence. 



The volk-plug now apparently recedes, leaving in its track a 

 groove from which the cells are proliferated. I have hitherto given 

 only the objective description of this process, and have not discussed 

 the nature of this apparent receding. I may now state that I believe 

 this is caused Ijy the active proliferation of cells from the lateral 

 blastopore lips, irliile pressiuff to meet each otlier in the median line. As 

 cells proliferated from the lips accummulate, they form a mass which 

 necessarily pushes the yolk-plug backwards. This pushing goes on 

 until the yolk-plug is placed at about the edge of the embryonic area. 

 The edges of the groove (or track, as I have called, it, left in the wake 

 of the yolk-plug) must therefore be considered the lateral blastopore 

 lips. Woodcut X, A, B, G will make this clear. A is the stage in 

 which the yolk-plug has not yet l)egun its backward, movement ; in 

 B, it has moved some distance and, in C it has reached its final 

 position. In the last stage, the blastopore therefore consists of three 

 parts : — 



(1) the dorsal opening of the neurenteric canal. 



(2) the groove between (1) and (3). 



(3) the groove around the yolk-plug in its final position. 



With regard to the first of these three ])arts, the dorsal opening of the 

 neurenteric canal (transformed from the blastop<jric passage, which, 

 in its turn, is directly derived from the posterior part of the archenteron) 

 must of necessity be considered as a part of the blastopore, but it would 

 be difficult in actuality to point out exactiv where the dorsal openin(j 

 lies, as it is merged beyond recognition in the posterior part of the 

 medullary canal, though that it is there, is a matter of course. IW most 



