Il8 N. M. STEVENS. 



Wilson's recent hypothesis ('09) that in some way two X 

 chromosomes determine the female sex, and one X chromosome 

 or an X and a F chromosome determine the male sex would 

 apply perfectly to Anopheles. 



Egg^T+sp. X=9, 

 EggX+sp. F=d\ 



But what shall we say of Culex and Theobaldia where there is 

 no differentiation of X and F chromosomes? In forms which 

 have an equal pair of heterochromosomes, showing the same 

 general characteristics as the unpaired heterochromosomes and 

 the idiochromosomes, one naturally infers that the members of 

 the equal pair in the male have X and F values respectively, 

 but shall we go further and argue that likewise in Culex and Theo- 

 baldia the mature eggs all have an X chromosome and the sper- 

 matozoa are dimorphic, one half having an X chromosome and 

 the other half a F chromosome, although nothing of the kind 

 can be traced? If we make this assumption, then Culex and 

 Theobaldia may stand at one end of a series, as examples of 

 absence of heterochromosomes associated with sex, while forms 

 with an unpaired heterochromosome come at the other end of 

 the series, and all the varieties of paired heterochromosomes 

 between, the mechanism of sex determination being the same 

 in all, but evident to the eye only when differential heterochro- 

 mosomes are present. 



The case of the aphids and phylloxerans has been the strongest 

 argument for the hypothesis that two X chromosomes give a 

 female and one X or XY a male, since the rejection of one X 

 chromosome from a parthenogenetic egg is followed by the devel- 

 opment of a male, but in Culex and Theobaldia it is evident that 

 sex determination is not dependent on the presence of X and F 

 chromosomes, although in Anopheles these chromosomes are at 

 least closely correlated with sex-determination. 



At present, the all-important questions seem to me to be: 

 What is the meaning of the differentiation of heterochromosomes 

 in one form and not in others closely related? W T hat has been 

 the history of such differentiation where we have an unpaired 

 heterochromosome or an unequal pair of heterochromosomes? 



