SEX IN A WASP 379 



division for X and Y, as it sometimes is in mammals. Further, 

 occasional parthenogenetic diploids arise (Speicher, 1934) pre- 

 sumably by fusion of the second division products and these are 

 always female, that is XY, never XX or YY like the biparental 

 males. 



The ordinary haploid males have their first meiotic division 

 suppressed as in Apis, the centrosome at one pole being separated 

 in a bud. The diploid males undergo the same process (Torvik- 

 Greb, 1935) and therefore produce triploid daughters (Bostian, 



1936). 



It is important to distinguish between the haplo-diploid system 

 of sex differentiation found in so many different groups of organisms 

 and the special XY mechanism of Habrobracon. Not only does the 

 haplo-diploid system extend to a large group of animals ; it also 

 has important physiological implications. It demands the develop- 

 ment of a special balance within all the chromosomes similar to that 

 which MuUer (1932) inferred in the X alone in Drosophila, viz., a 

 balance such that particular genes have the same effect in double 

 as in single dose. The development of this system to its present 

 perfection indicates its antiquity. On the other hand, the particular 

 XY mechanism shows some signs of newness. Whiting has repre- 

 sented the genie constitution of the differential segments of X and Y 

 as F.g and f.G so that they would be complementary. Crossing- 

 over still occurs between X and Y and gene differences are 

 exchanged. The differential segments are not, therefore, developed 

 to the extreme condition. Since neither in X nor Y are they ever 

 homozygous (except in the diploid males), it seems likely that they 

 never could develop except in the direction of inertness, and the 

 fact that the two kinds of males do not differ in any observable 

 respect bears out this expectation. It seems, therefore, that this 

 system is probably short-lived, capable of being frequently replaced 

 by similar systems, and probably secondary to the general haplo- 

 diploid method of differentiation. 



(c) CocciD^. In the coccids Pseudococcus ($, 2n = 10) and 

 Protortonia (?, 2« = 6) the haploid number of bivalents are formed 

 in oogenesis and meiosis is normal (Schrader, 1923, 1931). In the 

 male Pseudococcus (2m = 10) five of the chromosomes condense 



