END OF PROPHASE 103 



of the nucleus and at the end of prophase are extruded into the 

 cytoplasm, where they degenerate (Frankel, 1936, on FritiUaria ; 

 cf. Catcheside, 1934, on Brassica, and 1933, on CEnothera). 



Where, as in Pristiurus (Riickert, 1892) and Scolopendra (Bouin, 

 1925), considerable size variations occur in the nucleus during 

 prophase, the chromosomes vary in size proportionately, probably 

 through their retaining the same water-content relative to the 

 nuclear sap. They occupy, whether in an organism with large or 

 small chromosomes, less than one-hundredth of the space of the 

 nucleus. 



The relationship between the volume of the nucleus and of the 

 chromosomes in different organisms is perhaps not so variable as 

 would appear from a comparison of illustrations of this stage in 

 organisms with chromosomes of different sizes. 



But while this is true of sperm and pollen, and even of embryo- 

 sac mother-cells, it is not true of the very large egg-cells of animals. 

 In these the nuclei are ten or a hundred times as large as in the 

 sperm mother-cells. W^e then find a very instructive difference in 

 distribution. In the small nuclei the separate bivalents are evenly 

 distributed in the diakinesis nucleus. Where they are few in number 

 and much condensed this means that they lie chiefly on the peri- 

 phery. 



In the large nuclei, on the other hand, they are not evenly spaced. 

 They lie in a haphazard group. The repulsions between them which 

 are effective in giving an even distribution at short distances are 

 ineffective at longer distances. 



In some organisms the bivalents themselves continue to show no 

 special internal changes. Successive loops between chiasmata lie 

 at right angles to one another, but no general distinction can be 

 made between them in regard to size {e.g., Vicia Faba, FritiUaria 

 imperialis). In others one loop is distinguished from the others 

 by the sharp repulsion between its arms — so sharp that they are 

 drawn out into fine threads as they are afterwards at late metaphase 

 (Fig. 34). In these it is often found that the chiasmata are now 

 no longer interstitial — they have been gradually pushed along to 

 the ends of the chromosomes. All connection between the 

 chromosomes may then seem to be broken by the strong repulsion. 



