INTERSEXES AND SUPERSEXES 385 



although we can assume proportion from what we know of un- 

 balanced forms in plants. This is provided by work on forms of 

 Drosophila with different numbers of chromosomes. 



Bridges (1922) found that, in the progeny of triploid females of 

 Drosophila, flies occurred having different numbers of X and Y 

 chromosomes and autosomes (Fig. 121). Some of these were 

 sterile, but the whole of the types found might be arranged in a 

 more or less linear series from forms of male character to others of 

 female character, through normal sexual types and intersexes. 

 The extreme types at both ends were sterile and were described as 

 super-males and super-females ; those apparently normal males 

 which had no Y chromosome were also sterile, because Y, although 

 almost inert, still contains something necessary for fertility in the 

 male (Stern, 1929). The succession in this series was found to be 

 related to the proportions in which the autosomes and the X 

 chromosome were present, and entirely unrelated to the presence 

 of the y. Thus if the autosome complement is denoted by A, the 

 series (cf. Fig. 121) : — 



(AAAXXX {3x) ? 

 AAAX(Y), AAX(Y) ^, AAAXX{Y) \ AAXX {2x) ? AAXXX 



[aX(x)^ 



represents the transition from " super-male " to " super-female." 

 Haploid patches of tissue (AX) in mosaics have since been found to 

 be female in character as expected on this theory. 



It is interesting to notice that types otherwise similar are more 

 female with the higher number of fourth chromosomes. The fourth 

 chromosome therefore acts in the same sense as an A" chromosome, 

 and in the opposite sense to the other autosomes. This departure 

 from the rule can be simply understood by supposing that the small 

 fourth chromosome has arisen by fragmentation of the X, a not 

 improbable origin on general grounds. 



When the principles of proportion-adjustment or balance to be 

 derived from the simpler study of trisomic plants are applied to an 

 organism with alternative X and Y inheritance it is seen that two 

 systems of adjustment must work in such an organism. X and Y 

 must both be adjusted in proportion to the autosomes — as we have 



R.A. CYTOLOGY. 13 



