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This result is still more strikingly shown in cases where 

 the X-element consists of two or more components. Whatever 

 be their number in the diploid groups of the male — two in 

 Syromastes or Fitchia, three in Prionidus or Sinea, four in 

 Gelastocoris — twice this number appear in the female groups. 

 Thus the general formulas X + X = Female and X + Y = Male 

 (in which Y may = zero) are applicable to the chromosome- 

 combinations characteristic of the two sexes throughout this 

 whole series of species, whatever be the composition of the 

 X-element (fig. 2). 



This is hardly the place to trace in detail the history of 

 the subject or to enter upon the controversies to which it 

 has given rise. It may, however, be recalled that the first 

 basis for the cytological results was laid by Henking, in the 

 work already cited, by his fundamental discovery of the 

 dimorphism of the spermatozoa in Pyrrhocoris, though he did 

 not recognise the meaning of the phenomenon. The results 

 of this remarkably able investigator, long neglected, were first 

 confirmed by Paulmier in Anasa, 1 and later by many others. 

 The fruitful suggestion that the two sexes arise upon fertilisa- 

 tion of the eggs by the two respective classes of spermatozoa 

 is due to McClung, 2 who ably supported his view by a series 

 of interesting arguments. Unfortunately, however, no evidence 

 was at that time available in regard to the female chromosome- 

 groups, and McClung's conclusions therefore necessarily re- 

 mained of a somewhat hypothetical character. As the event 

 showed, his view was correct in principle ; but from a priori 

 considerations he was led (naturally enough) to regard the 

 X-class of spermatozoa as male-producing. The direct and 

 decisive evidence, first produced by Stevens and Wilson in 

 1905-6 3 by a detailed comparison of the male and female diploid 

 chromosome-groups, showed this view to be the reverse of 

 the fact ; and the results of these authors have since been 

 confirmed by many others, among whom may be mentioned 

 Montgomery, Boring, Lefevre and McGill, Wassilieff, Gutherz, 

 Morgan, von Baehr, Jordan, Davis and Morse. Boveri's recent 

 extension of the result to the nematodes has been mentioned 



1 Journ. Morph. 1899, 15, Suppl. 

 1 Biol. Bull. 1902, 3. 



3 See a series of papers by each of these authors in the Journ. Exp. Zool. with 

 literature there cited. 



