228 RIOHAKD ASSHETON. 



The effect up to now has been to produce a fairly sharp line 

 of demarcation between small and large cells upon the surface 

 at the point x in diagram^ fig. 15. 



On the supposition that this diagram represents fairly accu- 

 rately the distribution of yolk, it is clear that as this line 

 advances it encounters greater and greater resistance. May 

 not a time come when it will find the path of least resistance 

 to be inwards and backwards, as in diagram 16? 



Diagrams 15 and 16 are inaccurate for later stages of seg- 

 mentation, because they do not show a segmentation cavity. 



Fig. 12 is a more accurate representation of a completely 

 segmented egg. 



A, is the black upper pole (the anterior wall of the future 

 embryo); P. is the white lower pole (posterior end of the 

 future embryo) ; sff. the segmentation cavity. 



The letter A. points to the smallest cells of this stage, y-p. 

 to the largest. 



There is a gradual merging of the one into the other, not 

 along the surface, for here the line is much sharper, but along 

 the cells to which y. and w. are directed. 



My idea is that the continuation of the segmentation 

 process is the conversion of, first, the cells y., then the cells x. 

 into smaller ones, and in this way a layer of small cells will be 

 produced lying up against the mass of much larger cells y. p. 

 This layer I have indicated in the fig. 12 by the dotted line. 

 If a section of an embryo of the stage in which the blasto- 

 pore is nearly complete is examined, it will be seen that there 

 is such a layer of cells along the floor of the segmentation 

 cavity. 



Fig. 21 is an outline camera drawing. Such details as are 

 shown were not drawn by camera. The smallest cells are 

 those forming the lip of the blastopore. 



The point a. represents the at present most anterior limit of 

 the archenteron. More anteriorly, however, following the 

 lines a a., a a., there is what I take to be a differentiation of 

 the yolk-cells, that is a splitting up into smaller cells, which 

 cells, upon the splitting hypothesis, will eventually form the 



