DEVELOPMENT OF THE ALBINO RAT 23 



but one cell, as a rule, rises slightly higher than the other. This 

 is more clearly seen in reconstructions than in sections. In 

 figure G are shown reconstructions of the 2-cell stages, figured in 

 R and C of figure 1. In B, of the figures, the plane of section is 

 at right angles to the vertical axis of the reconstruction as shown 

 in B of figure G, while in C of figure 1, the plane of section is 

 parallel to the vertical axis of the reconstruction shown in A 

 of figure G. The equivalence or non-equivalence of the first 

 two blastomeres of the segmenting mammalian ovum has been 

 the subject of discussion since the time of Van Beneden's funda- 

 mental observations on the segmentation of the ovum of the rab- 

 bit. This discussion has been summarized a number of times 

 in recent years, and need not be entered into here. Suffice to 

 say that the consensus of opinion of the more recent contributors 



Fig. 6 Models, obtained by reconstruction after the Born method, of the 

 2-cell stages of the albino rat. Rat No. 59, 2 days. X 200. 



is, that the first two blastomeres of the mammalian ovum are 

 equivalent in size and structure if the stage is observed soon after 

 its formation. As above stated, the 2-cell stage of the mam- 

 malian ovum extends through a relatively long period, probably 

 about 24 hours. The two cells do not as a rule divide synchron- 

 ously, the division of one preceding the other by some little time, 

 resulting in a 3-cell stage. The cell to divide first increases 

 slightly in size and presents a clearer protoplasm prior to its 

 division. In a 2-cell stage, viewed in this phase of cytomorphosis, 

 one of the cells appears slightly larger with clearer protoplasm 

 than does the other cell, explaining the difference in size and 

 structure observed by Van Beneden and by other observers who 

 concur in his views. I am convinced that a difference in the 

 size of the two cells may be accounted for by the plane of section 

 in which they are cut, even though the nuclei of both cells are 

 included in the section. In the figures of sections of the 2-cell 



