400 EVOLUTION, GENETICS, AND EUGENICS 



general form in organisms as diverse as bryophytes, nematodes, echi- 

 noderms, arthropods and vertebrates is beyond a doubt of far-reaching 

 significance, and may be as widely distributed as Mendelian heredity 

 generally." While the same general scheme holds for all forms that 

 have been investigated, there exist many interesting differences in the 

 details of operation of the sex-determining machine. Some of the 

 simpler variations of the process are as follows: 



a) Variations of the Y -chromosome. — Beginning with a condition 

 such as that described for Drosophila, in which the Y-chromosome is 

 larger than the X-chromosome, there is a long series of species in 

 which the Y-chromosome becomes smaller and smaller until it dwindles 

 away to nothing and the male chromosome condition becomes XO in- 

 stead of XY. In the females of such species the condition remains XX. 



b) Variations of the X-chromosome. — In a number of species of 

 animals the X-chromosome may be represented by from two to nine 

 components, each of which at times has the appearance of a separate 

 chromosome. In a species of roundworms, Ascaris canis, for example, 

 the diploid chromosome number of the female is thirty-six and that of 

 the male is thirty, the difference being due to the fact that there are 

 two sets of six X-components in the female and only one set in the male. 

 In the reduction division of the male germ cell, the six X-components 

 all go in a group to one gamete and none to the other, so that two 

 kinds of gametes are produced, one with eighteen chromosomes and 

 the other with twelve chromosomes. AU the female gametes have 

 eighteen chromosomes. Apart from the fact that the X-chromosome 

 is in six pieces instead of but one, the mechanism of sex determination 

 is the same as it is in a group that has but one X-chromosome. 



c) Linkage of sex chromosome with autosome. — In a great many 

 species of insects the X-chromosome has been found to be imited to 

 one end of one of the autosomes, never losing this relation during the 

 entire chromosome cycle. Apart from this apparently secondary 

 union with an autosome, the behavior of the X-chromosome is the same 

 as in the XO cases described above. Hence the mode of sex deter- 

 mination is in line with the types already discussed. 



d) Female digamety. — In this mode of sex determination two differ- 

 ent kinds of eggs are produced, while the sperms are all alike. In other 

 words, there is simply an exchange between the sexes of the nuclear 

 differences characterizing males and females. Thus in the Lepidoptera 

 (butterflies and moths) the females have either the XY or the XO type 

 of chromosome complex, while the males always have the XX condi- 



