340 RIOHAED ASSHETON. 



than twice as thick as the ab-embryonic pole, instead of being 

 six or seven times as thick as it is in fig. 17. 



PI. 25, fig. 9, is an outline drawing of the embryonic pole of 

 this specimen. The nuclei of the outer layer are shaded with 

 lines, the nuclei of the inner mass with dots. The solitary 

 nucleus of the outer layer which overlies the inner mass is 

 smaller than the others. The inner mass is there seen to con- 

 sist of six cells only. 



Boundaries of cells cannot be distinguished at this stage, 

 except in silver nitrate specimens. 



I have obtained a large number of specimens between the 

 stages represented by my figs. 16 and 20. In these I find a 

 great inconstancy in the relative size of the inner mass to the 

 outer layer. Although the inner mass may consist of as few 

 as six cells, it is more usually of greater size than this. I 

 cannot say whether there is a reduction in the number of 

 cells of the inner mass after it has become separated ofi" 

 from the outer layer. 



Heape (14) described an occurrence of this nature in the 

 mole, and assumed that cells passed from the inner mass into 

 the outer layer. 



Just before the destruction of the zona radiata comes about 

 the inner mass shows a marked increase in activity, and 

 increases in size and becomes more compact (figs. 19 and 20). 



I have not seen in any specimen anything to indicate, before 

 the rupture of the zona radiata, which cells will become the 

 definite epiblast and which the hypoblast. The diameter of 

 the whole blastodermic vesicle just before the rupture is 

 about "15 mm. Until the rupture the blastocyst is spherical. 

 I cannot say how the rupture is effected. The zona radiata 

 becomes exceedingly attenuated, and I am inclined to think 

 that it becomes torn and broken in many places, and is not 

 thrown off in large pieces. 



Unless I have missed the intermediate stages, the dissolu- 

 tion of the zona radiata is accompanied by very marked 

 changes in the embryo. It loses its spherical form, and is 

 no longer tensely distended. The nature of its cells is 



