SEGMENTATION OF THE OVUM OF THE SHEEP. 213 



and stain more lightly, and occupy a more central position. 

 The darker smaller cells, which partially surround the inner 

 core of lighter cells, number twenty-four. The segments are 

 not now so spherical as they were ; they do not leave spaces 

 between each other, but are indeed pressed closely up against 

 each other (v. fig. 8). The difference in texture is exactly the 

 same as it was in the case of the eight-segment stage described 

 above (fig. 7). The light inner cells are left exposed over 

 about one fifth of the surface. 



The next stage is, I believe, represented by an embryo 

 obtained from the uterus of a sheep of six days. A median 

 section is represented in fig. 9. Here, again, there can be no 

 doubt about the different nature of certain of the segments. 

 A layer of darkly stained cells, which are on the whole 

 rather smaller, completely surrounds a group of lightly stained 

 cells. The light cells no longer appear on the surface at any 

 point. The small dark-staining cells number thirty-three, 

 and the inner light-coloured cells are five in number. Two 

 of these, however, are so nearly divided that one may say they 

 number seven. The outer layer of dark cells is decidedly 

 thicker at one point than elsewhere. In the preceding figure 

 (fig. 8) the middle cells of the outer layer are just a little 

 larger than those at the edge. If these two thickened parts 

 correspond, it may be concluded that the part of the inner 

 core which remained longest uncovered is that part imme- 

 diately opposite the thickened part of the outer layer. 



My fig. 10 represents the middle section of a series through 

 a specimen taken from the same sheep as that represented by 

 fig. 9. It was taken from the other horn of the uterus. This 

 specimen is made up of forty-four small dark cells and nine or 

 ten rather larger inner light cells. The outer dark layer of 

 cells has become doubled at one part, which I think we may 

 presume to be the part which had previously shown a thicken- 

 ing (fig. 9). It was stained with hsemalum, which for the 

 purpose of differentiating between the two kinds of cells is less 

 favorable than some other stains. It would seem to be in about 

 the same stage as the preceding specimen (fig. 9), or perhaps 



