IO J. E. WODSEDALEK. 



The difference in the interstitial cells is quite obvious. They 

 are much larger, much more numerous, and more regularly dis- 

 tributed in the horse than they are in the hybrid. The nuclei in 

 the former are large and usually contain one or two large nucleoli, 

 and several small karyosomes; while those of the mule are small 

 and contain many small karyosomes, and the large nucleolus is 

 not as constant as it is in the horse. The cytoplasmic content 

 of these cells in the horse, too, is larger and not as variable in size 

 as in the mule. 



SPERMATOGONIA. 



The spermatogonial cells of the mule are considerably larger 

 than those of the horse in the corresponding stages. They gener- 

 ally occupy the usual position next to the tubule wall, though oc- 

 casionally they are crowded out of this position by the numerous 

 nurse cells and pushed further in toward the lumen. They occur 

 in relatively fewer numbers in the hybrid than they do in the 

 horse. In cross sections of the tubules they very seldom form a 

 complete ring within the tubule wall, and are never found in a 

 crowded condition neither along the entire tubule wall nor in 

 small areas, as is often the case in normal mammalian tissue. The 

 cells are usually far apart, and it seems safe to state that in a 

 large majority of the tubules only about twenty to thirty per cent, 

 of their inner surfaces is covered with these cells. It seems 

 equally as correct to say that in cases where the spermatogonial 

 cells are most abundant they never exceed covering more than 

 sixty per cent, of the tubule's inner surface; and it might be said 

 that such extreme cases are rarely found. Many of the tubules 

 contain only a comparatively few spermatogonial cells and not 

 infrequently tubules are found in which no cells can be identified 

 as such. The cells occupying the position usually occupied by the 

 spermatogonia greatly resemble the ordinary nurse cells. 



During the prophase of the spermatogonial cell the nucleus is 

 round and contains a large nucleolus which is usually heart- 

 shaped, several small karyosomes, a large number of small 

 granular masses and extremely thin linen strands (Figs. I and 2). 

 In this respect the cells resemble those of the horse in the cor- 

 responding stages, except that they are much larger. No centro- 

 some can be detected at this stage although a dense mass of 



