590 SCIENCE PROGRESS 



of plant spores, that the male-producing cells " may be charac- 

 terised by the absence of some element that is present in the 

 female-producing ones. 1 If this be correct, a common principle 

 may apply to all cases, whether the germ-cell develops with the 

 haploid or the diploid number of chromosomes, and in the latter 

 case whether the heterogametic sex be the male or the female. 



In accordance with these considerations Castle, abandoning 

 the earlier form of his hypothesis, has endeavoured to show 

 how the facts may be given a general formulation in terms of 

 the "presence and absence" hypothesis of Mendelian heredity. 2 

 His suggestion is that the female is in all cases characterised by 

 the presence of something that is absent in the male, but two 

 general cases must be distinguished as follows : 



A. In the first case, where the male is heterogametic, the 

 presence of two X-elements in the zygote means the female 

 condition, the absence of one of them the male. 



B. In the second case the presence of one X-element means 

 the female condition, its absence the male. This assumption 

 may apply to cases where the female is the heterogametic sex ; 

 and it will explain such cases as the dioecious plants, where the 

 asexual spores may produce either males or females. 



The postulation of two such classes, while it gives a formal 

 explanation of the facts, is of course a speculative construction 

 ad hoc, and as such can for the present only be taken in a very 

 provisional way. It should not be forgotten, however, that 

 the cytological evidence establishes the actual existence of two 

 cases, one of which (insects) certainly corresponds to Castle's 

 Class A, while the other (sea-urchins) may correspond to 

 Class B. In any event the assumption offers some very interest- 

 ing suggestions for further inquiry, especially on the cytological 

 side. It seems at first contradictory to assume that in one 

 case a single X-element stands for the male condition, in the 

 other case for the female ; but this is not out of harmony 

 with known facts of heredity. For example, in the alternative 

 heredity of coloured forms when crossed with white, the presence 

 of colour usually " dominates " its absence, but the reverse case 

 is not uncommon. According to Shull 3 this may be interpreted 

 to mean that in the first case the presence in the heterozygote of 

 one colour-producing factor or "gene" is sufficient to produce 

 the colour, but in the second case colour only appears when 



1 Op. tit., Science, 1909. 3 Op. tit., 1909. 3 Am. Nat., July 1909. 



