SEX DETERMINATION 387 



Y neutrality implies balance. In the X recent work has indicated an 

 internal differentiation. Certain fragments of the X which have a 

 phenotypic effect on the males that bear them do not reduce their 

 fertility (Muller, 1930 a), probably on account of a margin of safety, 

 a threshold value, in the reaction of the normal type. WTien such 

 fragments are added to intersexes they alter the sexual grade of the 

 intersex in various degrees according to the fragment concerned 

 (Dobzhansky and Schultz, 1931 ; cf. Patterson, Stone and Bedichek). 

 These effects can be expressed more or less quantitatively as Gold- 

 schmidt has supposed. At the same time the X is shown to be 

 differentiated and adjusted, i.e., balanced (cf. Muller, 1932 a; 

 Kerkis, 1934). 



Proportion can be shown to be important in sex determination 

 in other organisms. Triploids and tetraploids in Rumex acetosa 

 and R. montanus of the constitutions : 18 + 2X + ^1^2 ^.nd 

 24 + 3X + 2Y1Y2 are intersexual (Ono and Shimotomai, 1928 ; 

 Ono, 1930), but whether those lacking Y^ and Yg are also intersexual 

 is not yet known. Forms with an abnormal (probably deficient) 

 autosome have, however, been found, and they, although diploid, 

 are slightly intersexual (Ono, 1930, 1935 ; Yamamoto, 1934). In 

 SphcBYOcarpus Donnellii (Knapp, 1935, a and h) gametophytes 

 lacking a certain part of the X chromosome are male, although they 

 have no Y chromosome at all. Intersexes in Melandrium album 

 have the male complement (Belar, 1925), while those in M. nibrum 

 have the female complement (Akerlund, 1927). These results 

 indicate one essential conclusion (which is in accordance with genetic 

 evidence) : all the chromosomes together establish the physiological 

 system which permits of two sharply distinguished lines of develop- 

 ment. The different proportions of the two sex chromosomes which 

 result from this segregation and recombination are merely the 

 trigger, as Muller puts it (1932), which sets development going in 

 one or other of the two directions. How the trigger mechanism 

 can be changed is shown by certain recent experiments which we 

 shall now consider. 



New mechanisms of sex segregation have arisen under two 

 conditions, either in hermaphrodites which had no such segregation 

 previously, or in sexually differentiated organisms where the new 



13—2 



