SEX DETERMINATION 



217 



rri factors in the different races and thus obtain a consistent explanation 

 of the whole series of hybridizations. The geographical distribution of 

 the races is fairly regular and suggests that the strength of the sex 

 factors may actually be an adaptive character related to the length of 

 the seasons, since a particular strength is probably an expression of a 

 particular speed of development (p. 272). 



In discussing the nature of the intersexual animals and the mechan- 



Fig. 100. Intersexes in Lymantria. — The intersexes arise in crosses between local 

 races which differ in the strengths of their cytoplasmic [Tl (female) and chromo- 

 somal M (male) factors. Some of the combinations are shown below; the male is 

 homogametic. 



F^ Males 



Cross 



weak ? X strong <^ 



half-weak $ x very strong <? 



half-weak ? x strong <J 



half-weak $ x medium <J 



neutral ? x any male <^ 



0. 



Mw Mstr. 



males 



[HhwMhwMy. 

 males 







hw Mhw Mstr 



males 



ll]hw Mm Mn 



males 



S 



neut M M 



males 



F^ Females 



Bw Mstr. 



males 



iZjhw Mv.str 



near-male intersexes 



Sw Mstr. 

 medium intersexes 



IZIhwMm 



low-grade intersexes 



B neut M 



females 



Note that the first cross gives only males in the F1, the females being com- 

 pletely transformed. From the last cross, with neutral $ x weak ^, intersexual 



males appear in F2. These are [Tj neut MwMw, when the [71 neut is too strong 



to be balanced by the weak M factors. 



ism of their development, it is first necessary to make a distinction 

 between intersexes and gynandromorphs. The latter (cf. p. 85) are 

 animals in which different parts have different genetic constitutions as 

 regards sex. They may be formed, for instance, by the loss of an X 

 chromosome from a female, perhaps at the first division of the egg, 

 giving a bilateral gynandromorph, or in later stages, and even several 

 times in the same animal, giving a more complex mosaic. An intersex 

 differs from a gynandromorph in that all its cells have the same genetic 

 constitution; it is not a mixture of genetically male and female parts, 

 but is genetically wholly of an intermediate character. 



Goldschmidt's first discovery about the nature of the intersexes was 

 that although they are not genetic mosaics, they are phenotypic mosaics. 



