12 J. E. WODSEDALEK. 



in size and shape (Wodsedalek, '14), and the same is probably 

 the case among the chromosomes of the ass. And the fact that 

 some of the chromosomes of the ass are apparently of the same 

 size or about the same size as some of the chromosomes of the 

 horse, makes it rather difficult to discriminate all of the maternal 

 from the paternal chromosomes in the hybrid. It appears, 

 however, that most of the large chromosomes in the mule are 

 paternal in origin. A careful study of the spermatogenesis of 

 the ass should throw considerable light on this subject. 



It is also definitely known, as was stated before, that the horse, 

 at least of some breed or breeds, possesses thirty-six chromosomes 

 besides the accessory, or sex-determining chromosome, in the 

 spermatogonial cells. The reduced number of the ordinary 

 chromosomes in this animal is eighteen, which number enter 

 into the formation of the one type of sperm, and eighteen plus the 

 accessory or nineteen in the other type. Since the spermato- 

 genesis of the horse with regard to the accessory (Wodsedalek, 

 '14) bears such a striking resemblance to the spermatogenesis of 

 the pig, in which animal the problem of sex-determination was 

 carefully worked out (Wodsedalek, '13), the author in his paper 

 on the spermatogenesis of the horse assumed that the sperm cells 

 possessing eighteen chromosomes are male determining and those 

 possessing nineteen are female determining. It was further 

 assumed on account of the behavior of the accessory in the horse 

 that the oogonia of the dam contain thirty-eight chromosomes, 

 or thirty-six autosomes and two accessories, while the mature 

 ova contain the reduced number of nineteen chromosomes, or 

 eighteen autosomes plus one accessory. 



The above figures are in all probability correct, but the actual 

 count of the chromosomes in the female tissue must be made 

 before it can be regarded as authentic, regardless of the strong 

 evidence obtained in favor of the assumption through the com- 

 parison of the behavior of the chromosomes in both the male 

 and female germinal and somatic cells of the pig. Great as the 

 similarity is, the mere fact that the pig is the only case among 

 the vertebrates in which the relation of the accessory chromo- 

 somes to sex-determination has been clearly and completely 

 demonstrated, any remarks based on such a comparison must 

 necessarily be of a more or less reserved nature. 



