SPECIAL CASES OF SEX-INHERITANCE 245 



statement of factors being carried by chromosomes, he 

 has only to say, that if the factors for the characters 

 follow the known distribution of the sex chromosome, 

 the results can be accounted for. 



The culmination of the evidence of sex-determina- 

 tion in man is found in a study of the cell structure 

 of the human race itself. Strange as it may seem, we 

 have been longer in doubt concerning the number 

 of chromosomes in man than in any other animal as 

 extensively studied. Four conditions are responsible : 



(1) The large number of chromosomes present in man. 



(2) The clumping or sticking together of the chromo- 

 somes. (3) The difficulty of obtaining fresh material. 

 (4) The possibility that the negro race has half as many 

 chromosomes as the white race. 



Two years ago Guyer announced the discovery that 

 in all probability there exist in man two unpaired 

 chromosomes in the male (Fig. 119) that behave in all 

 respects like that in the typical cases of the sort in 

 insects, where, as we have seen, there are two classes 

 of spermatozoa, differing by the addition of one more 

 chromosome in one class. These produce females ; the 

 lacking class produces males. But Guyer's evidence 

 was not convincing. He found in all 12 chromosomes 

 in one class of sperm and 10 in the other. Mont- 

 gomery has also studied the same problem, but his 

 account, while confirming the number, is in disagree- 

 ment in regard to the accessory. 



Jordan has gone over a number of other mammals, 

 in some of which he thinks that he has found indica- 

 tions at least of two classes of sperm. 



Still more recently another investigator, von Wini- 



