32 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



and polocytes contain 12. There is thus one extra chromosome in each mature 

 ovum and in each of half the spermatozoa. This chromosome is larger than the 

 others in some insects, and is termed the accessory chromosome. McClung was 

 the first to assume that the accessory chromosome was a sex determinant. It has 

 since been shown by Wilson, Davis, and others that the accessory chromosome 

 carries the female sexual characters. When the spermatozoan with 12 chromo- 

 somes fertilizes an ovum, the resulting embryo is a female, its somatic nuclei 

 containing 24 chromosomes. An ovum fertilized by a sperm cell containing only 

 n chromosomes (without the accessory chromosome) produces a male with so- 

 matic nuclei containing only 23 chromosomes. Winiwarther (Arch. d. Biol. Bd. 

 27) has recently made similar observations on the human germ cells but they have 

 yet to be confirmed by other investigators. It is probable, however, that sex is 

 transmitted by the human chromosomes in much the same way as in insects. 



