224 EVOLUTION, GENETICS, AND EUGENICS 



tion. Though the cytological evidence is still incomplete, it is prac- 

 tically certain that birds have the same peculiar method of chromo- 

 somal sex determination as the Lepidoptera, for they have the same 

 type of sex-linked heredity as the latter and the opposite of that seen 

 in mammals and most insects. Apart from the change of the digametic 

 condition from one sex to the other, the mechanism remains the same. 



Sex chromosomes in parthenogenesis. — When it became known 

 that parthenogenetic species (those in which eggs are capable of de- 

 veloping without fertilization) in some cases produce males and in 

 other cases produce females from parthenogenetic eggs, this seemed to 

 be out of accord with the theory of the chromosome mechanism of sex 

 determination. It is interesting to know, however, that, now that we 

 know the histories of the chromosome cycles in these species, the facts 

 are not only fully in accord with the chromosome theory, but greatly 

 strengthen it and enlarge its range of applicability. Two kinds of 

 parthenogenesis are known, which may be designated diploid and 

 haploid. In the former, the developing egg and embryo has the full 

 somatic number of chromosomes; in the latter, only half the somatic 

 number characteristic of the species is present. 



a) Diploid parthenogenesis. — In these species only one maturation 

 division occurs, and this division is not the reduction division; hence 

 each egg retains the diploid number of chromosomes, including two 

 X-chromosomes (XX). The result is that all eggs that behave in this 

 way develop into females. Thus in aphids and phyloxerans many suc- 

 cessive generations . of all females are produced. After a series of 

 female generations, a mixed generation appears in which males are 

 produced parthenogenetically along with females, but from smaller 

 eggs. Examination reveals the fact that male-producing eggs have, 

 after maturation, two less chromosomes than the female-producing 

 eggs. This was explained by the observation that when the first 

 maturation takes place, two chromosomes (obviously consisting of a 

 double X-element) are cast out into the polar body, while all the auto- 

 somes and two of the X-chromosomes remain in the egg nucleus. In 

 this way the male produced from this egg comes to have only two 

 X-chromosomes, while the female has four. This is really the equiv- 

 alent of XX for the female and XO for the male. In gamete forma- 

 tion the males produce two kinds of gametes, one with the double 

 X-element and the other with no X-element. Only the former of 

 these is viable; and this accounts for the fact that all fertilized eggs 

 produce females, for both gametes supply double X-elements. This 



