COMMENTS ON THK ORIGIN AND FA^OLUTION OF SEX 343 



One of rhc most interesting results of Cleveland's studies was 

 the finding of a differentiating nuclear division in the gametocytes of 

 haploid flagellates such as Trichonympha and Leptospironympba dur- 

 ing wiiich male and female vsets of chromosomes are segregated from 

 each other. Each parental chromosome gives rise to a chromatid of 

 each sex. 



The existence of sex-differentiating mitoses was postulated by 

 Prokofieva-Belgovskaya (1946). Starting with binucleate cells in the 

 bark parenchyma of potatoes, in which, after aging, two sister nuclei 

 come to have different appearances and behavior, then citing exam- 

 ples from the literature, this author presents some interesting interpre- 

 tations. The two sister nuclei in the potato cells, by the end of starch 

 formation, become considerably shrunken. The basophily of the 

 chromonemata and chromomeres is greatly diminished. In many cases 

 these sister nuclei differ from each other considerably, for example 

 in number of nucleoli and in stainability. The heterocyclicity of these 

 nuclei is also shown when they divide. The differences between the 

 two nuclei in their resting and divisional conditions are interpreted 

 to indicate that the mitosis which produced them was a differentiating 

 process. In anaphase, one set of chromosomes is supposed to be 

 "younger" and is segregated from the other, which is "older"; that 

 is, chromatids derived from the same parental chromosomes are not 

 identical. In line with some of the more recent ideas about gene and 

 chromosome duplications (for example, Delbriick, 1941), it is postu- 

 lated that a new unit forms as a "copy'' of the original instead of the 

 two units being formed by "splitting." Thus we have the concept 

 of "mother" chromosomes producing "daughter" chromosomes by a 

 sort of budding, and the latter may differ somewhat from the 

 "mother." These supposed differences in molecular makeup are 

 thought to account for the differences in appearance and behavior 

 of the sister nuclei described above. 



As a matter of fact, Cleveland's description of the separation of 

 male from female chromatids in the gametocyte division of Tricho- 

 nympha and other species indicates a mechanism that results in the 

 separation of one group of sister chromatids from the other. A sys- 

 tem of joining into two groups operates to prevent random orienta- 

 tion of pairs of chromatids and to insure the separation of all the 

 "male" from the "female" chromatids. Apparently this process is not 



