No. 2.] THE DEVELOPMENT OF THE NEWT. 333 



"For during the formation of the blastopore the epiblast does 

 not grow over the yolk-cells, enclosing them by a process of 

 epibolic invagination" {p. 461). While I think there is good 

 reason for holding that the process of ectoblast formation is not 

 one of simple overgrowth, but is modified by the advancing 

 differentiation of cells /;/ siUi, I do not see any reason for the 

 wholesale denial of such overgrowth. There can be no question 

 that the roof of the segmentation cavity contains much more 

 material in the early stages than in the later (cf. Figs. 46-48). 

 The disappearance of such a mass of substance may be best 

 accounted for by supposing that cell proliferation has carried 

 it further towards the lower pole. In fact, as I shall state 

 presently, there is conclusive evidence that such overgrowth 

 does occur. Robinson and Assheton, furthermore, fail to see 

 that denial of this epibolic invagination involves them in serious 

 difficulties when they attempt to explain the closure of the 



blastopore. 



I have fortunately succeeded in obtaining decisive ocular 

 evidence that the small cells around the lips of the blastopore 

 are actually infolded. With the aid of the underlying plane 

 mirror previously mentioned, I have repeatedly watched in the 

 living egg the slow rolling in of the small epiblast cells. This 

 I have seen in the newt and in Rana palustris, and, through 

 the kindness of Mr. A. C. Eycleshymer, I have repeated his 

 observations upon Ambly stoma piinctatwn, where the large 

 size of the cells renders the demonstration peculiarly convin- 

 cing. The small cells roll down over the others (epibolic 

 invagination), and at the same time the cells around the edge 

 of the blastopore turn in and disappear from view (embolic 

 invagination.?). It is obvious that the appearance of embolic 

 invagination may be in part delusive. The small cells at the 

 rim of the blastopore may simply remain stationary, while the 

 other cells grow down over them. In other words, it may not 

 be wholly cell proliferation that forms the " invaginated hypo- 

 blast," but a simple modification of the process of epibole. In 

 the one case, the -invaginated hypoblast" is active, in the 

 other, passive. I do not at present see how it is possible to 

 differentiate sharply the two processes, and distinguish between 



