SEX 



103 



representative of each of the 7 pairs, 14 chromosomes 

 in all. But when the male forms gametes the odd 

 chromosome does not divide. It passes over entire 

 to one of the sperms, the other lacking it altogether. 

 Consequently, two classes of sperms are found in 

 equal number, viz. those possessing 7 and those 

 having only 6 chromosomes. When a sperm con- 

 taining 7 chromosomes fertilises an egg the complete 

 quota of pairs is established, and the resulting in- 

 dividual is a female. But when an egg is fertilised 

 by a sperm containing only 6 chromosomes, the in- 

 dividual that results contains but 6 pairs and an odd 

 one, 13 chromosomes in all, and is a male. In such 

 cases the sex of the individual is evidently determined 

 by the presence of both members, or of only one, 

 of this particular pair of chromosomes. For reasons, 

 however, which may now be given, it is customary to 

 speak of them as the X chromosomes. In the case 

 of Protenor the X chromosome has no pair in the 

 male. But in certain other species it has a pair, 

 but a pair which differs in size and shape from itself. 

 Generally the pair to the X chromosome in the 

 male is smaller than the X chromosome itself, and 

 can be readily distinguished from it. When present 

 it is termed the Y chromosome. In such cases the 

 constitution of the female is XX and all of her eggs 

 contain an X chromosome. The male is ^K in 

 constitution, and half of the sperms he produces 

 contain an X chromosome, while the other half 

 contain a Y chromosome. Half of the offspring are 

 XX^ viz. females ; the other half XY, males. 



In DrosopJiila anipelopJiila, the species of pomace 

 fly with which Morgan and his co - workers have 



