May, 1905.] The Nature of the Reduction Division. 



339 



As stated above, Mendel's law can be explained on the theory 

 of pure sex cells. In working out the peculiar activities of the 

 chromatin during cell division, cytologists have come to look 

 upon the chromosomes as special bearers of hereditary tendencies 

 although other parts of the protoplasm may also have something 

 to do with the transmission of heredity. Now if a transverse 

 division occurs and the chromosomes are pure the daughter 

 nuclei could then be organized as pure having only chromosomes 

 derived from the egg or sperm (Fig. 5, a-b). No difference how 

 many subsequent, longitudinal splittings take place before the 

 formation of gametes, the gametes would always be pure cells. 



a b c d 



Fig. 5. Diagram of transverse division showing possible production of 

 pure and mixed cells. 



In conjugation there is twice the chance for a mixed oospore to 

 be formed as a pure one and hence the splitting of the hybrid 

 race in the proportions given by Mendel's law. But suppose 

 that the chromosomes were joined in pairs and arranged in the 

 mother star in such a way that half of the male chromosomes 

 were on one side and half on the other and the same 

 for the female chromosomes then the transverse splitting 

 would always result in mixed cells and no splitting of the race 

 could occur (Fig. 5, c-d). The daughter nuclei would be mixed 

 even if the chromosomes making up the pair were pure. Other 

 arrangements are possible, and in case the chromosomes are not 

 reorganized as pure bodies the cells resulting from reduction 

 could of course not be pure. But whatever the facts may be it 

 appears that all cases of hybrids that follow Mendel's law as well 

 as those which do not can be accounted for on the theory of pure 

 chromosomes and a qualitative reduction division. This would 

 not prove however that the chromosomes are organized as pure 

 bodies or that there is a transverse splitting of chromosomes. 

 These facts must be worked out from a study of nuclear division, 

 and this is the important and difficult problem to be solved. 

 Anyone can compare the results of cytology and plant and ani- 

 mal breeding after the facts have been ascertained. But to work 



