THE CELLULAR BASIS 



143 



egg contains one large accessory chromosome. 

 If such an egg is fertilized by a sperm con- 

 taining a large accessory (the X chromosome) 

 it gives rise to a female, if by a sperm contain- 

 ing a small accessory (the Y chromosome) it 

 gives rise to a male (Fig. 35). 



In other animals one may not be able to 

 distinguish separate X or Y chromosomes and 

 yet such structures may be joined to one or 

 two ordinary chromosomes. This is the case 



Oocyte 



Reduotio 

 Divisia 



Spermatoeyte 



FIG. 35. DIAGRAMS OF SEX DIFFERENTIATION IN THE BEETLE, 

 Tenebrio, showing 5 synaptic pairs of chromosomes (there are 

 actually 10 pairs) ; in the oocyte the members of each pair are 

 equal in size; in the spermatocyte the members of one pair 

 are unequal. These pairs separate in the reduction division 

 giving rise to two types of spermatozoa and one type of ova; 

 eggs fertilized by one type of sperm give rise to females, those 

 fertilized by the other type give rise to males. (After Stevens 

 with modifications.) 



