296 J. E. WODSEDALEK. 



In the last spermatogonial cells the chromosomes appear in a 

 dense mass. The cells gradually increase in size and great expan- 

 sion takes place in the nuclei (Figs. 5-9). The chromosomes 

 become decidedly distinct and surprisingly well segregated 

 throughout the spherical nucleus (Figs. 10 and n). The cells 

 in these stages are numerous and beautiful. The chromosomes 

 appear to be dense in structure and are thicker than those of the 

 early spermatogonial cells. Several of the chromosomes are 

 almost spherical. They are so evenly distributed that hundreds 

 of accurate counts can be made within a short time. This 

 condition prevailed in all of the mature testes studied and the 

 cells were especially numerous in three two-year-old bulls. Even 

 in somewhat stale tissue (Fig. 12) the chromosomes in these 

 cells appear to remain well segregated. Altogether over one 

 thousand accurate counts were made in these cells alone. 



2. Primary Spermatocyles. 



The sex-chromosome can invariably be seen in the spermato- 

 cytes where it retains its individuality (Fig. 13). Just how 

 pairing takes place in these cells cannot be stated with certainty. 

 When the chromosomes appear for division they are of the 

 reduced number and bivalent in nature. The thirty-six ordinary 

 chromosomes pair while the sex-chromosome remains unpaired 

 and can easily be distinguished from the others (Figs. 14-18). 

 It occasionally shows its double nature in the late prophase 

 (Fig. 17), and more frequently in the later stages of the primary 

 spermatocyte division (Figs. 22, 24, 25, 27, 28 and 29). 



During the primary spermatocyte division the sex-chromosome 

 usually passes to one pole in advance of the other chromosomes 

 (Figs. 19-25). This unequal division of the chromosomes in these 

 cells (Figs. 19-29) gives rise to two different types of secondary 

 spermatocytes. The one type containing the eighteen ordinary 

 chromosomes plus the sex-chromosome. Just before division is 

 complete the chromosomes become loosely paired (Figs. 28 and 

 29). This peculiar behavior of the chromosomes is apparently 

 quite common in mammalian tissue. 



