296 J. E. WODSEDALEK. 



followed out it might lead to serious misinterpretations. Since 

 the occurrence of the chromatoid body in the horse is so constant 

 and its behavior so distinct, and furthermore, since this is the 

 first case among the vertebrates where such a body has been 

 studied in full detail, it is dealt with at some length in this paper. 



The significance of the accessory chromosome is of course 

 obvious. It was shown beyond doubt that sex in the pig is 

 determined by such elements (Wodsedalek,' 13). And while 

 embryological material of the horse is not at present available 

 to enable a similar extended study, the presence and unquestion- 

 able behavior of the accessory chromosome giving rise to a di- 

 morphic condition among the spermatozoa of this mammal, 

 nevertheless, lend additional support to the chromosome theory 

 of sex determination. The spermatogenesis of the horse re- 

 sembles to a certain extent the spermatogenesis of the pig 

 (Wodsedalek, '13), and for the purpose of avoiding too much 

 repetition it is treated in a comparative way in the present study. 



This investigation was started in the zoological laboratory 

 at the University of Idaho, but the main bulk of the work was 

 done at the Wisconsin Biological Station at Madison. And I 

 wish to thank the zoology department of the University of Wis- 

 consin for the liberal use of their laboratories, apparatus and 

 material, and the many other courtesies extended me during 

 the summer of 1914. 



II. MATERIAL AND METHODS. 



The material studied, mainly, was obtained from a horse 

 about a year and a half old. Immediately after the testes were 

 removed from the live animal, small pieces were placed in 

 Bouin's and Gilson's fluids. Sections from various parts of the 

 testes were made from four to ten microns thick, and the material 

 fixed in Bouin's fluid and stained with Heidenhain's iron hema- 

 toxylin with acid fuchsin as a counterstain, as in the case of the 

 pig, proved to be the most satisfactory. 



Material from an animal about a year old was also studied; 

 but w T hile all the stages including the mature sperm could easily 

 be identified in this material, the chromosomes were very difficult 

 to count on account of being too closely aggregated or lumped 



