SEGMENTATION OE THE OVUM. 



57 



of the equatorial plate now divide probably by splitting longitudinally so that 

 the number of chromosomes is doubled. During the splitting the chromosomes 

 tend to draw apart from one another. At the same time the spindle, without 

 changing its length, becomes somewhat narrower. The chromosomes now move 

 apart from the equator toward the two poles, forming two groups, each group 

 containing half of the total number of chromosomes (Fig. 1 1), and at the same 

 time the whole ovum becomes somewhat elongated in the direction correspond- 

 ing with the axis of the spindle. The chromatin granules accumulate at the two 

 poles of the spindle. The achromatic threads between the poles break through. 

 Then the actual cleavage of the elongated ovum into two cells becomes marked 

 in the protoplasm, and the line of separation of the two cells passes through the 

 equator of the spindle. The accumulated granules of chromatin then reconsti- 

 tute the resting membranate nucleus (Fig. 12). In brief, the segmentation of 



FIG. 12. OVA OF WHITE MOUSE WITH- Two SEGMENTATION SPHERES OR CELLS. 



A, Telophase of the division ; the chromosomes are reconstituting the nucleus. , Membranate nucleus recon- 

 stituted. /, First cell of segmentation. nu, Nucleus, p.g, Polar globules. Z, Zona pellucida. X 5 

 diams. (After Sobotta.) 



the ovum is a typical indirect or mitotic cell division. In the mouse the first 

 cleavage is completed about twenty-six hours after the coitus. The second 

 cleavage is not completed until twenty-four hours later. When first formed, two 

 segmentation spheres are oval and entirely separated from one another, but 

 subsequently they flatten against one another and become appressed, a phe- 

 nomenon of which we have no explanation. In most mammals which have been 

 studied there is more or less space between the segmenting ovum and the zona 

 (see Fig. 8), but in the mouse this space is reduced to a minimum and the zona 

 is often stretched into irregular forms during the changes of the ovum. 



The succeeding cleavages of segmentation need to be followed out in greater 

 detail than yet recorded. In many cases there appear to be three cells in the 

 next stage, because one of the two primitive segmentation spheres divides sooner 

 than the other. The more commonly received view is that four cells are pro- 



