The Cellular Basis 



159 



tained the accessory chromosome, and he pointed out that if an 

 egg were fertilized by a sperm containing an accessory, two ac- 

 cessories would be present in the zygote, this being the condition 

 of the female, while if it were fertilized by a sperm without an 

 accessory there would be present in the zygote only the accessory 

 derived from the egg (Fig. 52 E and F, Fig. 53). 



XY Type. In other cases Miss Stevens as well as Wilson dis- 

 covered that t'wo accessory chromosomes, differing in size, might 

 be present in the male whereas in the female they are of equal 

 size (Fig. 54). In such cases two types of spermatozoa are pro- 

 duced in equal numbers, one containing a large and the other a 

 small accessory chromosome, whereas every egg contains one 

 large accessory chromosome. If such an egg is fertilized by a 

 sperm containing a large accessory (the X chromosome) it gives 

 rise to a female, if by a sperm containing a small accessory (the 

 Y chromosome) it gives rise to a male (Fig. 54). 



Oocyte 



SpermcLtocyte 



Fertilized 

 Egg 



Speririatida 



FIG. 54. DIAGRAMS OF SEX DETERMINATION IN THE BEETLE, Tcncbrio, 

 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 sper- 

 matocyte 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.) 



