The Cellular Basis 



159 



chromosome more than those formed from the other daughter cell 

 (Fig. 51 C-F). 



Chiefly because these two kinds of spermatozoa 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 accessory 

 chromosome, there was only one type of egg, since every egg con- 

 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 



Pratenor Type 



Oocyto 



Bmrmatooyte Kedvcti6n 



Sp/armaAids 



FIG. 53. DIAGRAMS OF SEX DETERMINATION 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, two are present in the oocyte, but only one in the sperma- 

 tocyte. In the reduction division the synaptic pairs separate, giving rise to 

 two types of spermatids, one of which has the sex chromosome and the 

 other lacks it ; all ova are alike in this regard. 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.) 



