SEX-CHROMOSOMES II 



is the same, it would happen that there would be an approxi- 

 mately equal number of these two kinds of offspring. We 

 know that the only quality which separates the members of 

 the species into these two groups is that of sex.' 



Thus it was that the chromosome complex came to be 

 associated with sex-determination. McClung's main hypo- 

 thesis was complicated by its association with a subsidiary 

 one of selective fertilization which led him to the conclusion 

 that the spermatozoon carrying the extra chromosome was 

 male-determining. If this were so, then the male had to be 

 the sex which had one chromosome more than did the 

 female. Sutton (1902), by reporting that the spermatogonia 

 of Brachystola possessed one chromosome more than did 

 the ovarian follicle cells, provided support for McClung's 



error. 



The work of McClung aroused great interest and much 

 controversy. It evoked great activity in the field of cytology. 

 Gross (1906) claimed to have demonstrated that in Syro- 

 mastes and Pyrrhocoris the accessory chromosome (the 

 single, unpaired one) present in the spermatocytes arose 

 from two small spermatogonial chromosomes and, further, 

 that the number of chromosomes was the same for both 

 sexes. It was his opinion that all spermatozoa lacking the 

 accessory chromosome degenerated so that only one type of 

 functional gamete remained. However, Stevens (1905) m the 

 beetle Tenebrio and Wilson (1905) in the bug Lygaeiis 

 furcicus showed clearly that in these forms at least there was 

 one pair of unequal chromosomes and that this pair behaved 

 in the growth stages of the spermatocytes exactly like the 

 unpaired accessory chromosome. They found also that the 

 members of this unequal pair separated and passed to oppo- 

 site poles in one of the two meiotic divisions. Stevens further 

 demonstrated that the diploid number of chromosomes was 

 the same in both sexes but that in the female no pair con- 

 sisting of unequal mates was present, the male being XY, 

 the female XX. Then in 1909 Wilson corrected Gross by 

 showing that in the male of Pyrrhocoris there was an un- 

 paired chromosome in the spermatocyte and that this arose 

 from a single spermatogonial chromosome of corresponding 



