THE CELLULAR BASIS 147 



"odd" chromosome was left without a mate 

 (Fig. 32 B) . Later, in the reduction division, 

 when the synaptic pairs separated, the odd 

 chromosome went entire into one of the daugh- 

 ter cells, and the spermatozoa formed from 

 this cell contained one chromosome more than 

 those formed from the other daughter cell 

 (Fig. 32 CandD). 



Chiefly because these two kinds of sperma- 

 tozoa occur in equal numbers McClung in 

 1902 concluded that this accessory chromo- 

 some 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 the other lacked the ac- 

 cessory 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 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. 33 E and F, Fig. 34). 



