108 PETER OKKELBERG 



under practically identical conditions, it does not seem likely that 

 these variations can be due to environmental factors. 



Many mendelian workers have found it convenient to assume 

 that the appearance of the two sexes in approximately equal 

 numbers in most animals is due to the fact that one sex is hetero- 

 zygous for sex, while the other is homozygous for this character. 

 This suggestion comes from the fact that the sex ratio corre- 

 sponds with the ratio obtained when a first-generation hybrid is 

 bred to a pure recessive. In this case half of the offspring will 

 be pure recessives or homozygotic, while the other half will be 

 hybrid or heterozygotic with the dominant character present. 

 From the study of the sex chromosomes it seems to have been 

 found that one sex may produce two kinds of germ cells which 

 are visibly different, while the other sex produces only one kind. 

 It appears further that it is sometimes one sex and sometimes 

 the other that is heterozygous with regard to the sex character. 

 It has been supposed, and is still maintained by a number of 

 investigators, that the sex chromosomes are absolute sex deter- 

 miners. The idea, however, that the chromosomes act quali- 

 tatively has given way to the belief that the influence exercised 

 by the sex chromosome is a quantitative one, and this concep- 

 tion has paved the way for a better understanding of the sex 

 phenomena in forms that exhibit hermaphroditic tendencies. 

 The conception that all individuals carry the factors of the oppo- 

 site sex in a latent condition will probably prove to be correct, 

 and it may lead to a general acceptance of the theory that sex is 

 not unalterably fixed at the time of fertilization. 



The primary difference between a male and a female in any 

 species is not as great as one might conclude from the appearance 

 of the adults of the two sexes. The first sexual changes usually 

 take place in the germ gland; in the female some of the germ 

 cells very early stop dividing and enter upon a period of growth, 

 while in the male the germ cells continue to multiply for a long 

 time. 



Since the primary difference between the female and the male 

 is, that in one the germ cells enter early upon a period of growth, 

 while in the other they continue to divide, it seems probable that 



