SEGMENTATION OF THE OVUM OF THE SHEEP. 233 



be of insufficient evidence to prove the descent of the light 

 and dark cells of the later stages from those similarly coloured 

 in the earlier figs. 8, 9, 11. This would be in accordance with 

 Duval's view. 



But if I attempt to take this interpretation, and say that 

 the dark cells of figs. 8, 9, 11, are all epiblast and the light 

 cells hypoblast, I find that, as may be seen in the figs. 12 — 

 15, there is no tendency whatever for the dark cells to 

 become grouped together into a solid mass, sucb as the em- 

 bryonic epiblast is in the later stages (figs. 1 7, 19) . On the con- 

 trary, the darker cells of the inner mass in every section show 

 a looser and a more peripheral arrangement to the inner mass. 

 I find it is not possible to trace either the dark outer layer 

 cells of fig. 11 into a compact inner mass, or to trace the 

 lighter inner mass cells of fig. 11 into a loose investing 

 sheath as at fig, 17. Whereas for the reverse course there 

 is, as I have shown in the earlier pages of this paper, not 

 a little evidence. 



Is it possible to put an interpretation upon the ontogeny of 

 the bat similar to that which I believe should be placed on the 

 ontogeny of the sheep, such as suggested in my diagrams 

 (V— V7 on PI. 18) ? 



I believe that a closer examination of Duval's and van 

 Beneden and Julin's works indicates that such an interpre- 

 tation is possible, as I will now attempt to show. 



The account given by Duval is by far the most complete 

 description of the development of any bat hitherto published. 

 In some most important features it differs from that given by 

 van Beneden and Julin (13). 



The authors are, however, agreed that there is from the 

 first division of the ovum a distinct difference, by which the 

 segment which will ultimately give rise to the epiblast cells 

 may be recognised from that which will give rise to the hypo- 

 blast. 



They agree that the four-segment stage consists of two 

 larger spheres and two smaller spheres. 



Van Beneden and Julin say that two are smaller^ darker. 



