EARLY STAGES OE DEVELOPMENT OP THE RABBIT. 127 



to find differences of any value between the inner and outer 

 cells. The same may be said of sections of specimens treated 

 with silver nitrate. 



Fig. 16 is a drawing through the centre of a specimen from 

 a rabbit seventy-two hours after coition^ fixed with nitrate of 

 silver ^ per cent., exposed to the light and stained with picro- 

 carmine. 



Summary up to the Seven ty -second Hour. 



The original description of the segmentation stages by 

 Bischoff I believe to be in the main correct. I cannot find 

 any evidence to support van Beneden's view of the origin of 

 the inner mass of cells from a smaller, more darkly staining 

 primary segment only ; or for the origin of the outer layer of 

 cells from a larger, more lightly staining primary segment 

 only ; or, again, for the growth of the descendants of one of 

 the two primary segments round the descendants of the other 

 primary segment. 



It must be borne in mind that the very fact of van Beneden's 

 description of the later stages of development (the origin of 

 the mesoblast and hypoblast from the inner mass, and that of 

 the epiblast entirely from the outer layer) having been shown 

 to be wrong by several authors (Kdlliker,^ Heape, Balfour, &c.), 

 made it almost certain that van Beneden's description of the 

 segmentation stages was incorrect, or, at any rate, that the 

 interpretation he put upon the supposed facts could no longer 

 be held to be sound. 



My account is briefly as follows : 



1. The ovum about the twenty-fourth hour after coition 

 divides into two segments, one of which is usually larger than 

 the other, there being much variation in this respect. 



2. Each of these segments again divides about the twenty- 

 sixth hour after coition, each dividing very nearly at the same 

 time. These four segments now resulting may vary in size. 



3. The third series of divisions takes place about the twenty- 



' A. KoUiker, ' Festschrift zur Feier des 300 Jalirigen bestehens der Julius- 

 Maximilians-Universitat zu Wiirzburg,' 1882. 



