2 2 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



nothing but blue-eyed individuals. The first generation from the cross in (3) will have black 

 eyes solely, for black in the present example is dominant, as it is termed. Such black-eyed 

 individuals, nevertheless, possess blue-eyed bearing chromatic particles in their germ cells; 

 in the progeny resulting from the interbreeding of this class the original condition is repeated 

 — pure blacks, impure blacks which hold blue recessive, and pure blues will be formed in the 

 ratio of 1 : 3 : 1 respectively. It is thus seen that blue-eyed children may be born of black- 

 eyed parents, whereas blue-eyed parents can never have black-eyed offspring. Many such 

 allelomorphic pairs of unit characters are known. 



DETERMINATION OF SEX 

 The assumption that the chromosomes are the carriers of hereditary ten- 

 dencies is borne out by the observations of cytologists on the germ cells of inver- 

 tebrates, especially insects, and of some vertebrates. According to Winiwarter 

 (Arch, de Biol., T. 27, 1912) the nuclei of human spermatogonia contain 47 chro- 

 mosomes, while those of the oogonia contain 48. When maturation and reduction 

 of the chromosomes take place in the male cells, one unpaired chromosome fails 

 to divide and passes intact to one or the other daughter cells; hence half of the 

 spermatids contain 24 chromosomes, the other half only 23. All the oocytes and 

 polocytes, on the contrary, contain 24. There is thus one extra chromosome in 

 each mature ovum and in each of half the spermatozoa. This chromosome, 

 because of pecuharities of size or shape, can be identified easily m many animals, 

 and is termed the accessory chromosome. McClung was the first to asstune that 

 the accessory chromosome is a sex determinant. It has since been shown bv 

 Wilson, Davis, and others that the accessory chromosome carries the female 

 sexual characters. When, in the case under consideration, a spermatozoan -nith 

 24 chromosomes fertihzes an ovum, the resulting embryo is a female, its somatic 

 nuclei containing 48 chromosomes. An ovum fertilized by a sperm cell containing 

 only 23 chromosomes (without the accessory chromosome) produces a male -nith 

 somatic nuclei containing only 47 chromosomes. These ob '■-^ns of Wini- 



warter on man have yet to be confirmed by otLer investigators. It is probable, 

 however, that sex is transmitted by the human chromosomes essentially in the 

 manner described, which agrees with the easily observed phenomena in insects. 



