SEX AND PARTHENOGENESIS 383 



(1931) to be the most decisive evidence in favour of the theory that 

 this set is " progressively degenerating," but it seems rather to be 

 the chief obstacle to regarding the series of steps as progressive. 



If it were possible to assume that the sperm which survived 

 carried both kinds of chromosomes the series could be taken to 

 show a progressive change, and this would be the means by which 

 the whole of one set might be lost in the same way and for the same 

 reason as the Y chromosome has been lost in the Orthoptera and 

 elsewhere. Hence the haplo-diploid system of sex-differentiation, 

 which is found in yet another coccid, Icerya purchasi, might arise 

 (v. diagram). 



(v) Meiosis in Cyclical Parthenogenesis. Males arise in the 

 cyclically parthenogenetic Rotifers, as they do in the Hymenoptera, 

 presumably by haploid parthenogenesis. In the aphids and 

 phylloxerans it has been established (cf. Morgan, 1912, 1915 ; 

 Schwartz, 1932) that the development of males is due to the loss of 

 one chromosome (an X chromosome) at maturation in the egg of 

 the parthenogenetic mother. The 2a:— i males might be expected 

 to give offspring like themselves as well as females, but the true- 

 breeding parthenogenetic line is restored by the functioning of only 

 those sperms with the full haploid number. 



The same aberration occurs less regularly in the Orthoptera. In 

 Apotettix and Paratettix, Robertson (1930, 1931 c) found that the 

 males exceptionally produced by parthenogenetic females lacked 

 one of the X chromosomes of their parents, as do the normal males 

 in the grasshoppers. This was supposed to be the result of non- 

 disjunction, probably conditioned by a special property of the 

 X chromosomes. Such a property as, for example, that of precocious 

 development found in the single X chromosomes in the male might 

 affect its pairing if equally present in the female, and hence lead to 

 lagging and loss (v. Ch. IX). 



(vi) Genetics of Sex Determination. In any species where the 

 chromosomes of the two sexes differ in size the sex differentiation 

 can be seen to follow from the segregation of dissimilar chromosomes 

 at meiosis. In some species (as shown in Ancyr acanthus by Mulsow) 

 every step in development can be seen, from the act of unequal 

 segregation to the differentiation of the sex types which is invariably 



