146 PHYSICAL BASIS OF HEEEDITY 



discovered) and on this account one might still ascribe 

 failure to cross over in this pair to its peculiar condition. 



The interest in the situation became even greater when 

 it was found that in the silkworm moth (in which the sex 

 formula is reversed, so to speak) crossing over is again 

 absent in the sex that is heterozygous for the sex fac- 

 tors — here the female. The female moth is apparently 

 ZW, at least in two cases. 



In one of the flowering plants, Primula sinensis^ cross- 

 ing over occurs in both sexes (Gregory, Altenburg), but 

 the amount of crossing over in the pollen is somewhat dif- 

 ferent from that in the ovules. Gowen has examined 

 Altenburg 's data statistically and finds that the differ- 

 ence is probably significant. 



That crossing over should take place in the sex that is 

 homozygous for the sex-chromosomes (the female in 

 Drosophila, the male in the silkworms) but in both sexual 

 elements in the hermaphrodite plant (Primula) may 

 appear to have a deeper significance, but more recent dis- 

 coveries seem to deprive the results of any such meaning. 

 Castle, for instance, gives data that show crossing over 

 in the male rat (the male is probably heterozygous for 

 the sex-chromosome), and Nabours gives data for crossing 

 over in the male and female grouse locust, Apotettix 

 (in which the male is presumably heterozygous). Until 

 more cases are forthcoming it must seem doubtful, there- 

 fore, if any such relation as that mentioned above is a 

 general one. 



