Studies on Chromosomes 27 



the spermatozoa exists in Nezara, yet this condition is con- 

 nected, by an almost continuous series of intermediate forms, 

 with one in which a conspicuous difference of nuclear con- 

 stitution is to be seen. It seems hardly open to doubt that 

 sex-production conforms to the same essential type throughout 

 this series. At least a possibility is thus established that in 

 organisms generally both eggs and spermatozoa may be pre- 

 destined as male-producing and female-producing forms, whether 

 they are visibly different or not. In any case, it is evident 

 that in the Hemiptera the chromosome-combination characteristic 

 of each sex is established by union of the gametes and is a result 

 of fertilization by one or the other of the two forms of spermatozoa. 

 Sex must therefore already be predetermined in the fertilized egg, 

 and it is difficult to conceive how it could subsequently be altered 

 in these animals by conditions external to the egg or embryo. 

 Since the idiochromosomes or heterotropic chromosomes form the 

 distinctive differential between the nuclei of the two sexes, it is 

 obvious that these chromosomes are definitely coordinated with 

 the sexual characters. We must therefore critically inquire into 

 the causal relation between sex-production and the chromosomes, 

 of which this coordination is an expression. 



That sex-production may be interpreted as the result of a 

 Mendelian segregation, transmission and dominance of the sexual 

 characters has been shown by Castle ('03). The history of the 

 differential chromosomes in synapsis and reduction evidently 

 affords a concrete basis for such an interpretation in the terms of 

 the Sutton-Boveri chromosome-theory. Analysis of the facts now 

 known will, however, show even more clearly than the more 

 general considerations adduced by Castle, that this interpretation 

 is only admissible under the assumption that a selective fertiliza- 

 tion occurs, such that eggs containing the female-determinant are 

 fertilized only by spermatozoa containing the male-determinant 

 and vice versa. Until I had read Cuenot's recent interesting 

 paper ('05) on the breeds of mice and their combinations, the 

 necessity for making this assumption seemed to me an almost 

 fatal difficulty in the way of the interpretation, but if Cuenot's 

 conclusions be well founded the a priori objections to such a 



