THE CELLULAR BASIS 141 



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 C andD). 



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). 



In other cases Miss Stevens as well as Wil- 

 son discovered that two accessory chromo- 



