82 ZOOLOGY 



that much real light has been thrown on the subject. 

 When it came to be realized that the chromosomes were 



Drawing by R. >, cjer 



FIG. 18. Diagram to show spermatogenesis, the small "sex-chromosome" going to 



one sperm of every two. 



of prime importance in the study of heredity, these 

 bodies were scrutinized with great care, in a considerable 

 number of animals. It was found that the sperm cells 

 of certain insects were not all alike, but were of two 

 kinds, differing in the number of chromosomes. There 

 was a peculiar chromosome, often standing a little apart, 

 which existed in one of the kinds of sperm, not in the 

 other. It was evident that in the reduction division 

 this chromosome had no mate, and hence only one of 

 every two sperms could receive it. Consequently just 

 half the sperms possessed one more chromosome than 

 did the other half. Further investigations showed that 

 the peculiar chromosome, now called *, did often have a 

 mate, usually smaller, which was named the y-chromo- 

 some. In such cases every other sperm contained an 

 x, the rest a y. There were other modifications of the 

 scheme, but the general outcome was as follows : Each 

 of the egg cells contains an .r-chromosome ; when a 

 sperm containing an x unites with it, then the zygote 

 contains 2x and produces a female. When the sperm 

 lacks an #, then the zygote comes to have x or xy, and 

 produces a male. Although this was made out first in 



