FERTILIZATION OF OVUM IN WHITE MICE. 



165 



nucleus on the other attach themselves to the equatorial region of the spindle (Fig. 

 120). The spindle continues to grow; the chromosomes become V-shaped and ar- 

 range themselves as the so-called equatorial plate, in which the chromosomes of 

 the two pro-nuclei can no longer be distinguished from one another (Fig. 120). At 

 each end of the spindle is a distinct centrosome with a very faint, small astral 

 radiation in the neighboring protoplasm. This spindle is the beginning of the divi- 

 sion of what we may call the segmentation ,,..,,-..... ^i^^^^^^^.^i,^.,. . 

 nucleus. In the mouse the two pro-nuclei do 

 not actually fuse into a single nucleus before the 

 formation of the spindle, which initiates the first 

 division of the fertilized ovum, so that, strictly 

 speaking, there is no fusion of the pro-nuclei to 

 make a segmentation nucleus. There is, never- 

 theless, a true fusion of the pro-nuclei accom- 

 plished, although it is somewhat masked by the 

 early commencement of the first segmentation 

 spindle, which develops at the same time that the 

 fusion of the pro-nuclei is being completed. 



The chromosomes of the equatorial plate now divide, probably by splitting longi- 

 tudinally, 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 



FIG. 122. OVUM OF WHITE MOUSE. FIRST 

 SEGMENTATION SPINDLE. 



The chromosomes have divided and have 

 migrated toward the poles of the spin- 

 dle, forming two groups. X 1500 

 diams. (After Sobotta.) 



p.g. Z. 



FIG. 123. OVA OF WHITE MOUSE WITH Two SEGMENTATION SPHERES OR CELLS. 



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

 stituted. I, First cell of segmentation, nu, Nucleus, p.g, Polar globules. Z, Zona pellucida. X 500 

 diams. (After Sobotta.) 



containing half of the total number of chromosomes (Fig. 122), and at the same 

 time the whole ovum becomes somewhat elongated in the direction corresponding 

 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 



