REPRODUCTION 79 



only are the chromosomes thus concerned with 

 sex determination, but from the fact that they 

 possess somewhat different forms, there is rea- 

 son to believe that a particular chromosome is 

 responsible for this determination. Thus in 

 Protenor an especially large chromosome is 

 always found in those sperm cells that have 

 seven such bodies, that is, in those that pro- 

 duce females, and it is as regularly absent 

 from those that produce males. Hence this 

 chromosome is sometimes called the sex chro- 

 mosome. 



Sex determination from the point of view 

 just presented has been studied in many other 

 animals and, though the process presents a 

 great variety of details, it seems to turn reg- 

 ularly on the chromosome composition of the 

 reproductive cells. In man, as already stated, 

 the reproductive elements are not so favorable 

 for this kind of study as in many other ani- 

 mals, but within a few years Winiwarter has 

 shown that human spermatozoa, like those of 

 Protenor, are probably of two classes, one 

 characterized by the presence in each cell of 

 twenty-three chromosomes and the other by 

 twenty-four such bodies. If this is true, sex 

 determination in man is brought about prob- 



