Ill 



GERM-TRACK 



S^ 



9 



Now in the normal monospermic ^^g all the daughter chromosomes 

 of the zygote nucleus which pass into the one blastomere (designated by 

 Boveri, S) undergo diminution, and all that go into the other (P) undergo 

 their next mitosis intact. Thus, if the inducement to diminution were 

 furnished by the chromosomes themselves, we would have to suppose 

 that every chromosome of the normal zygote nucleus divides at meta- 

 phase into two daughter chromosomes, one of which is predestined to 

 undergo diminution and the other is not. If this were the case in the 

 dispermic egg, it would follow that six of the daughter chromosomes 



Fig. 40. 



Diagram illustrating the part played by the cytoplasm in determining the diminution of the chromatin 

 in Ascarts megalocephala. (After Boven, Feslschr. f. Hertwis, 1910.) The shaded portion represents the 

 vegetative cytoplasm In every case the chromosomes remain intact :n the cells containuig this 

 substance, and undergo diminution in the cells which lack it. A, undivided egg ; B, C, D, stages in the 

 production of the 4-cell stage in the normal, monospermic egg ; E, F. G, results of the first cleavage of 

 disperniic eggs. According to the orientation of the spindles with regard to the egg axis, one of the three 

 types shown is obtained. 



produced by the metaphase of the first cleavage mitosis were predestined 

 to undergo diminution, while their six sister chromosomes were not. 

 Moreover, it would often result from the unequal distribution of the 

 daughter chromosomes among the four primary blastomercs, that the 

 same nucleus would contain a mixture of chromosomes predestined for 

 diminution and of those predestined to remain intact. As a matter 

 of fact, neither of these expectations is reahzed. Diminution may take 

 place in one, two or three of the four cells formed by the first cleavage 

 division. All the chromosomes in any one nucleus behave aUke, and the 

 number of chromosomes which escapes diminution, instead of being 

 always six, varies from two to twelve in different eggs. 



