i6o 



Heredity and Environment 



derived from the egg (Fig. 52 E and F, Fig. 53). This "acces- 

 sory" chromosome was therefore called the "sex determining" or 

 merely the "sex" chromosome and was designated by the letter X; 

 consequently its double occurrence in the female was indicated by 

 XX; its single occurrence in the male by XO, the O standing for 

 zero or no chrompsome. 



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

 covered that two 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 with the formula XX, if by a sperm containing 



Oocyte 



Fertilized 



FIG. 54. DIAGRAMS OF SEX DETERMINATION 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 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.) 



