162 SEX-DETERMINATION [CH. 



which is different from, and commonly smaller than, the 

 corresponding body in the female. As a result the male 

 produces two kinds of spermatozoa, one containing a 

 chromosome-group exactly similar to that present in all 

 the eggs, and the other differing in respect of the sex- or 

 idio-chromosome. It remains to consider how these facts 

 can be related, firstly to the condition already described in 

 the Hymenoptera, secondly to the production of males and 

 females by parthenogenesis in such forms as the Aphids, 

 and thirdly to species in which hermaphroditism occurs. 



In the Hymenoptera, as has been described earlier in this 

 chapter, the male has the haploid (n) number of chromo- 

 somes at least in the cells of the "germ-track," while the 

 female has the diploid or 2n number. In consequence of 

 this fact the spermatocytes undergo no reduction division, 

 and the spermatids contain the same number of chromo- 

 somes as the spermatogonia. Obviously these facts can be 

 brought into line with the condition found in so many 

 other insects if it be assumed that one of the chromosomes 

 of the male is a "sex-chromosome" (hetero-chromosome) 

 and that therefore two of those of the female are sex- 

 chromosomes. If this assumption is made, it is clear that 

 the Hymenoptera are similar to the other insects described 

 in having a single hetero-chromosome in the male and two 

 in the female, but since there is no reduction in the sper- 

 matogenesis all spermatozoa will contain the hetero-chromo- 

 some and therefore all fertilised eggs will contain two and 

 will become females. From the facts of artificial partheno- 

 genesis it is known that a normal individual can develop 

 from an egg containing only the haploid number of auto- 

 somes, and it may thus be supposed that the Hymenoptera 

 offer only a special case of the condition found in so many 

 other insects, being similar in respect of the sex-chromo- 



