THE CELLULAR BASIS 



149 



small accessory chromosome, whereas every 

 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 



Oocyte 



Tenebrio Type 







Reduction 



Ferti 



lizcd 



Division Ova 





'Egg 



Spermatocyte Reduction 

 Division 



Spermatids 



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.) 



