390 PERMANENT HYBRIDS 



In the simplest cases of genotypic sex differentiation no differences 

 can be seen to distinguish any sex chromosomes {e.g., in the higher 

 plants, Spinacia, Haga, 1935 ; Taxus, Dark, 1932 ; in the Poly- 

 chaeta, Ophryotrocha, Huth, 1933, and in the Amphibia ; in eleven 

 out of thirty species of the Hepaticae, Lorbeer, 1934). Genetically 

 the same lack of differentiation is to be inferred in Lebisies and 

 Habrobracon. But where complex genetic differences distinguish 

 the two sexes we find complex differences between their chromo- 

 somes, either in their form or in their relative behaviour at meiosis. 

 Many observations bear witness to the importance of structural 

 change in the development of the sex chromosomes, e.g., fusion in 

 Mermiria, fragmentation in Phragmatobia, Metapodiiis, and Alydus, 

 translocation in Drosophila species, interchange in Humulus. 



The question then arises as to how a group of differences can act 

 as a permanent unit in inheritance since crossing-over is a general 

 property of pairing chromosomes, how, in other words, a 

 differential segment can develop. The most obvious explanation 

 is that the sex factors arise in association with structural changes. 

 Small relatively translocated or inverted segments evidently have 

 little chance of crossing-over. This hypothesis is an expression in 

 cytological terms of Muller's suggestion (1918) that the differences 

 between complexes in (Enothera and between X and Y chromosomes 

 in the sex heterozygote were due to the association of lethal factors 

 with crossing-over suppressors. We now know that structural 

 changes may combine these two functions (of which the first is 

 largely superfluous in sex differentiation). However, a variety of 

 evidence indicates that structural changes have not been the 

 primary agent in the suppression of crossing-over and the con- 

 ditioning of differentiation. 



Suppression of crossing-over might equally be supposed to be 

 genotypically determined. This suppression would permit the 

 development of structural differences which would themselves 

 secondarily prevent crossing-over. The evidence that this is the 

 origin of differentiation is both genetical and cytological, as 

 follows : — 



(i) Precocity of the sex chromosomes combined with pairing by 

 1 3rminal affinity, or precocity of parts of them, leading to localisation 



