RESULTS OF CROSSING TWO HEMIPTEROl'S SPECIES. 189 



Recent experimental results have caused a marked modifi- 

 cation of the views of some of the adherents of the more extreme 

 chromosome hypotheses, forcing them to modify the theory of 

 the individuality and continuity of the chromosomes so far as to 

 admit that there must be an interchange of chromatin between 

 individual chromosomes and that the chromosomes which emerge 

 from synapsis are "probably not identical with the original 

 conjugants" (Wilson, '12, pp. 422). They do not however 

 extend this interpretation to the XY chromosomes: "the degree 

 of union may vary in different cases, involving sometimes no 

 fusion, as is suggested by the history of the XY pair" (Wilson, 

 '12, pp. 417). This would seem to be an inevitable conclusion, 

 otherwise any facts that could be assumed to be explained on the 

 supposition of an interchange of factors between A' and Y, 

 would be inexplicable for those forms in which no Y is present. 



In analyzing the results of our recent experiments we shall 

 accept, for the sake of the argument, the above assumption that 

 there is no interchange of material between the XY chromo- 

 somes, and also the hypothesis of male- and female-producing 

 spermatozoa. 



In 1909 Castle suggested that the Y chromosome of those 

 forms in which this morphological element is present may be the 

 bearer of all characters that are exclusively male. He wrote: 

 " I would offer the suggestion that we have a mechanism suitable 

 for the transmission of characters exclusively male in the Y 

 element described by Wilson, the 'synaptic mate' of the A" 

 element." 



It would seem possible to test the value of this suggestion 

 by crossing two species, each having the XY chromosomes, and 

 one of the species having an exclusively male character which 

 is lacking in the other. These conditions are met in the two 

 Hemipterous species Euschistus variolarius and Euschistus servits. 

 The former has a character that is exclusively male, in the form 

 of a distinct black spot on the male genital segment, while such 

 a spot is entirely lacking on the male genital segment of Euschistus 

 servus (Photo i). There is no black spot on the female genital 

 segment of either species. 



During 1911 and 1912 we succeeded in crossing three species 



