CHROMOSOMES OF HUMAN SPERMATOCYTES 6 



Figures 17, 18, 19 and 20 represent the corresponding stage 

 in the negro. A careful study has revealed no essential differ- 

 ences in the two races. In figure 20 one of the chromosomes 

 has divided into ay and a 2. The justification for this inter- 

 pretation may not be entirely clear from the figure, but in the 

 actual section the chromosomes ai and 0-2 lie respectively con- 

 siderably above and below the level of the remaining chromo- 

 somes. In figures 18 and 19 the m chromosome has divided 

 into nil and //^2. In figure 17, 12 undivided chromosomes may 

 be counted. 



The chromosomes at this stage in either race show more or 

 less constant differences in form and size. The most constant 

 and conspicuous examples of this are seen in the large distinctly 

 bivalent body, the components of which are unequal in size 

 (XF, figs. 13, 14, 16, 17 and 18), and a small bivalent body m, 

 appearing as a short constricted rod with rounded ends as in 

 figures 12, 13, 14 and 15, etc., or as two spherules connected by 

 a slender strand as in figures 17 and 20. As has been noted 

 above, in figures 18 and 19 it has divided (7711 and 7no). 



Turning now to earlier stages, I have found abundant instances 

 of dividing spermatogonia, but very few favorable for accurate 

 counting. Figure 1 is a drawing of spermatogonia! late prophase 

 (white) in which 24 rod-shaped chromosomes may be counted. 

 Figure 2 represents the same stage in the negro and likewise 

 contains 24 chromosomes. The latter figure was drawm from 

 two sections. I have not found it possible to identify in these 

 groups the X and Y chromosomes. A pair of very small chromo- 

 somes may be seen in either plate which probably represent the 

 constituents of the bivalent rri chromosome of the primary 

 spermatocyte. 



The growth period is characterized by a very distinct bipartite 

 chromatin nucleolus closely applied to the nuclear membrane 

 and staining very intensely with the basic dye (figs. 4, 5, 6 and 

 7). Its size varies at different times in the growth period. Thus 

 in the early part of the period it is very difficult to distinguish 

 it in the tangled chromatin threads, but as growth proceeds it 

 becomes more and more prominent until finally it is the most 



