CAUSES OF STERILITY IN THE MULE. 13 



The investigations on the relation of the accessory chromosome, 

 or group of chromosomes, to sex-determination in at least another 

 vertebrate, the man, must not be overlooked. And while there 

 is some difference of opinion in regard to this matter in man and 

 the results are not entirely conclusive, they should nevertheless 

 add more strength to the above assumption. Guyer ('10) found 

 that the spermatogonia of man contain twenty-four chromo- 

 somes; and that twelve chromosomes appear for division in the 

 primary spermatocyte of which ten are evidently bivalent and 

 two accessories. During division ten chromosomes pass to one 

 pole and ten plus the two accessories to the other, giving rise to 

 two different types of secondary spermatocytes which eventually 

 give rise to two types of spermatozoa; the one type containing 

 ten chromosomes and the other ten plus the two accessories. 



Montgomery ('12) confirms the number of chromosomes found 

 by Guyer but disagrees with him in regard to the accessory. 

 Von Winiwarter found forty-seven chromosomes in the male, of 

 which forty-six unite at reduction and form twenty-three, bi- 

 valents and the accessory remains unpaired. Two types of 

 spermatozoa are produced, the one containing twenty-three 

 chromosomes and the other twenty-three plus the accessory, or 

 twenty-four chromosomes. In the female he had some difficulty 

 in obtaining the exact number of chromosomes, but his best 

 counts gave forty-eight, which number fits in with the results 

 obtained in the male. 



The cause of the difference of opinion is probably due to the 

 fact that Guyer used Negro material in his investigation, while 

 Von Winiwarter studied tissue obtained from a white man. 

 In view of the difference between these two human races it is 

 not at all improbable that a difference in the number of chromo- 

 somes also exists. The Negro is fully as far removed from the 

 white man as is the ass from the horse, where a great difference 

 in the number of chromosomes apparently exists. This vast 

 difference in number appears to be, at least in part, responsible 

 for the sterility of the mule. The difference between the white 

 man and the Negro in regard to the number of chromosomes 

 according to Guyer and Von Winiwarter is equally as great, but 

 unfortunately the mulatto is fertile. 



