164 Heredity and Environment 



negroes, removed in castration and fixed immediately by the most 

 approved methods. He finds that in both whites and blacks, there 

 are 48 chromosomes in the spermatogonia, or 24 synaptic pairs in 

 the first maturation division; one of these is plainly the XY 

 pair, the X and Y being unequal in size. I am indebted to Painter 

 for the privilege of using one of his unpublished drawings (Fig. 

 56A) and for permission to quote his conclusions. Guyer has also 

 informed me personally that in new and better material he now 

 finds 48 chromosomes in human spermaftogonia, one of these being 

 an X and another a Y. These discoveries appear to settle once 

 for all this vexing question and to establish the fact that in man, as 

 well as in many other animals, sex is determined by the chromo- 

 somes, the sex chromosomes being XX in the female, and XY 

 in the male. 



Sex a Mendelian Character Correlations Between chromo- 

 somes and sex have been observed in more than one hundred 

 species of animals belonging to widely different phyla. In a few 

 classes of animals, particularly Lepidoptera and birds, the evi- 

 dence while not entirely convincing seems to point to the fact 

 that two types of ova are produced and but one type of sperma- 

 tozoa; but the general principle that sex is determined by the 

 chance union of male-producing or female-producing gametes is 

 not changed by such cases. 



Sex, therefore, appears to be inherited, that is, its factors are 

 present in the germ cells but probably not as particular genes oc- 

 cupying definite loci in a chromosome but rather as a relation of 

 whole chromosomes, such as XX, XY or XO (see p. 172) ; it is a 

 Mendelian character in which the female is usually homozygous 

 for sex while the male is heterozygous. Consequently in the 

 formation of the gametes every egg cell receives one sex de- 

 terminer, while only one-half of the spermatozoa receive such a 

 determiner, the other half of them being without it. If then an 



* Painter, T. S. The Y Chromosome in Mammals, Science, May 27, 1921. 

 Also letter dated November 3, 1921. 



