78 BIOLOGY AND SOCIAL PROBLEMS 



expected, these chromosomes become reduced 

 in number to half that characteristic of the 

 cells of the body ; in other words, each mature 

 egg cell contains seven chromosomes. When 

 the ordinary cells in the body of the male of 

 this insect divide, the number of chromosomes 

 is seen to be thirteen, one less than in the 

 female, and when the spermatozoa in this ani- 

 mal are formed, the reduction in the original 

 number of thirteen results in the formation 

 of two classes of sperm cells, one with seven 

 chromosomes and the other with six. These 

 sperm cells are formed in equal numbers and 

 are apparently the means of sex determina- 

 tion. For, when an egg with its seven chromo- 

 somes is fertilized with a sperm having the 

 same number, an individual arises whose body 

 cells exhibit fourteen chromosomes, and such 

 animals are females. When, on the other 

 hand, an egg is fertilized by a sperm with only 

 six chromosomes, an individual whose body 

 cells contain only thirteen chromosomes re- 

 sults, and the animal is a male. It is thus 

 clear that in this instance the fertilizing sper- 

 matozoon determines the sex of the offspring 

 and that the material which is concerned in 

 this is that contained in the chromosome. Not 



