GEOWTH IN LENGTH OF THE FEOG EMBEYO. 229 



roof of the archenteron, and come to lie up against the epiblast 

 now forming the roof of the segmentation cavity, as shown 

 diagrammatically in figs. 12 and 13. 



The differentiation as seen at the point a a. must, on this 

 hypothesis, be considered to be the effect of the direct continua- 

 tion of the process of differentiation on the surface of the ovum 

 (whereby the epiblast is separated), that is a direct continua- 

 tion of the process of segmentation. This line bounded above 

 by small cells, below by large cells, constitutes a line of separa- 

 tion or a split, which is, I believe, the first commencement of 

 the archenteron, and is a result of the primary centre of 

 activity, comparable to the events of the first five days in the 

 development of the rabbit, or to the formation of the gastrula 

 in Amphioxus. But although corresponding in effects, the 

 only really homologous feature is the presence of a primary 

 centre of activity, or process of segmentation of the egg ; the 

 actual directive agencies being in each case coenogenetic and 

 entirely different. 



The conversion of the narrow slit into a spacious cavity is to 

 be considered to be due, at any rate in part, to the effect of the 

 secondary centre of activity, to which I shall refer again. 



I must now refer to the experiments made by Roux and 

 Schultze and Morgan and Ume Tsuda upon the developing egg 

 by following natural or artificial spots, which experiments I 

 have myself repeated during this spring. 



It is impossible to repeat these experiments without becom- 

 ing convinced that there is a change of relative position between 

 certain spots on the ovum, — for instance, the dorsal lip of the 

 blastopore, and the most inferior spot upon the white pole 

 of the ovum. These two spots, as seen from without, un- 

 doubtedly approach one another before the complete formation 

 of the circular blastopore. But the question to what extent 

 this approximation is carried;, and whether by a concrescence 

 of the lateral lips of the blastopore, or by a rolling under of the 

 white pole, or by a growth over the upper lip without concres- 

 cence, is answered differently by the several observers. 



My own suggestions are as follows. 



