The Cellular Basis 



similar condition in which one race has twice as many chro- 

 mosomes as another race of the same species is found in two 

 races of the thread worm, Ascaris megalocephala. However, more 

 recent work has shown that the number of chromosmes in white 

 men and in negroes is the same. Wieman found 24 chromosomes 

 in the spermatogonia ( ?) of both races and he inferred that in 

 the male there is an XY pair of sex chromosomes, in the female an 

 XX pair. On the other hand, Evans* found in hundreds of counts 

 that there were constantly 48 chromosomes in the spermatogonia 

 of a white man, thus indicating the presence of an XY pair of 

 chromosomes in the male and an XX pair in the female. 



The most recent work on the number of chromosomes in man 

 is by Painter,* who has studied normal testes of both whites and 



Mr* 



FIG. 56. SPERMATOGNIAL CHROMOSOMES OF NE*GRO. A. Spermatogonium 

 showing 48 chromosomes, the smallest of which is the sex chromosome Y; 

 the other sex chromosome X, is a small rod-shaped one. As reproduced 

 the figure is magnified about 3000 diameters. From a preparation and 

 drawing by Painter. 



B. The same chromosomes spread out and arranged, according to size 

 and shape, in 24 synaptic pairs. 



* See Babcock and Clausen, p. 538. 



