76 MARY T. HARMAN AND FRANK P. ROOT. 



more cells in these stages than the other stages. This is true, 

 not only for the animals used in this study, but also for other 

 guinea pigs which we have examined. 



In the early prophase of the primary spermatocyte cells the 

 chromatin is coarsely granular and is in an indistinct reticulum. 

 There is no uniformity in either the size or the shape of these 

 granules in a particular cell, but there are always a few which are 

 distinctly larger than the majority and decidedly more compact. 

 As the nucleus increases in size, the chromatin granules become 

 smaller and the reticulum less distinct until the nucleus is 

 almost clear, except for here and there masses of chromatin of 

 loose construction. After this the nucleus does not get much 

 larger but the chromatin material becomes more concentrated 

 near one side of the nucleus until it becomes a tangled mass in 

 which it is difficult to distinguish any particular parts or any 

 definite arrangement. Gradually this mass loosens up and more 

 nearly fills the entire nucleus. In addition to the reticulum there 

 is a number of more compact particles of chromatin material. 

 These seem to be the beginnings of the bivalent chromosomes. 

 Long before all the chromatin material has been formed into 

 definite chromosomes each of these masses appears double. 

 Fig. 3 shows nineteen of these double chromosomes with the 

 chromatin in so loose a condition that each body has a woolly 

 appearance. Fig. 5 is a later stage in which the chromatin is 

 more compact and the double condition is no longer apparent. 

 Figs. 4 and 6 are fully formed primary spermatocyte chromo- 

 somes. By reference to these figures it will be seen that no two 

 chromosomes are of exactly the same shape or size. As was 

 found in the spermatogonial divisions the spermatocyte chromo- 

 somes are distributed through the entire diameter of the spindle. 



3. Spermatocyte Divisions. 



Figure 7 is a lateral view of a metaphase plate of a primary 

 spermatocyte division. It will be noted that one chromosome is 

 almost divided and that the division is an unequal one. We have 

 thought that this is the XY-chromosome. We have designated 

 the larger component as the X and the smaller one as the Y. 

 Fig. 8 shows the autosomes undivided while the XY has com- 



