THE CELLULAR BASIS OF HEREDITY 



129 



Chiefly because these two kinds of spermatozoa occur in equal num- 

 bers McClung in 1902 concluded that this accessory chromosome was a 

 sex determinant. In 1905 Wilson discovered in a number of bugs that 

 while there were two types of spermatozoa, one of which contained, and 



Oocyte 



Protenar Type 



Reduction Ova 



Division 



Fertilized Egg 



Spermatocyte 



Reduction 

 Division 



Spermatids 



Fig. 36. Diagkams of Sex Differentiation in the Bug, Protenor. The oocyte 

 contains 6 chromosomes and the spermatocyte 5 chromosomes which are not yet 

 united into synaptic pairs ; the " sex " chromosomes are shown in black and white, 

 two are present in the oocyte, but one is present in the spermatocyte. In the reduc- 

 tion division the synaptic pairs separate, giving rise to two types of spermatozoa, one 

 of which has the sex chromosome and the other lacks it ; all ova are alike in this re- 

 gard. If an egg is fertilized by a sperm without the sex chromosome a male results ; 

 if fertilized by a sperm containing the sex chromosome a female results. (After 

 Wilson with modifications.) 



Tenebrlo Type 



Oocyte 



Recitation 

 Division 



Ova 



Ferti Lized 



Em 



Spermatocyte 



Reduction 

 pttft'giotv 



Spentiatids 



Fig. 37. Diagrams of Sex Differentiation in the Beetle, Tenebrio, showing 

 5 synaptic pairs of chromosomes (there are actually 10 pairs) ; in the oocyte all pairs 

 are equal in size ; in the spermatocyte one pair is 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.) 



the other lacked, the accessory chromosome, there was only one type of 

 egg, since every egg contained the accessory chromosome, and he pointed 

 out that if an egg were fertilized by a sperm containing an accessory, 

 two accessories would be present in the zygote, this being the condition 



