FOUNDATIONS FOR SEX 



also to the further complications which 

 arose with more extended study. These 

 complications included differences not only 

 in one chromosome pair but in several. The 

 significance of a balance between these 

 chromosome types as well as with the en- 

 vironment was grasped by Goldschmidt 

 and particularly by Bridges where his more 

 favorable material brought out sharper 

 contrasts in types and in the chromosome 

 behavior. Ideas related to chromosome bal- 

 ance as they may affect developmental 

 processes were developed. Goldschmidt em- 

 phasized that this balance could include 

 factors in the cytoplasm as well as in the 

 chromatin material. Bridges' observations 

 on the other hand, pointed most strongly to 

 the chromatin elements where changes in 

 chromosome numbers were often accom- 

 panied by sharp differentiation of new 

 sexual types. 



Concepts of sex determination broadened. 

 They came to include all the chromosomes 

 in the haploid set, a genome, as a whole. 

 The important concept of single chromo- 

 some difference being all important has 

 been replaced with that of a balance be- 

 tween the chromosomes. It has sometimes 

 been emphasized that it is this balance 

 which is the most significant element in sex 

 determination. Present day research seems 

 to be pointing rather to even finer struc- 

 tures than the chromosomes in that the 

 main search is on for the particular genes 

 within the inheritance complex which con- 

 tribute the characteristics of sex. Bridges' 

 work has sometimes been asserted as com- 

 mitted to a particular type of balance in 

 the chromosome. However, his writings 

 1 1932) make it clear that this balance is, in 

 his judgment, basically due to the genes 

 actually contained within the chromosomes 

 rather than to the chromosomes themselves. 

 This view is further emphasized in the 

 second edition of Sex and Internal Secre- 

 tions. "Sex determination in numerous 

 forms with visible distinctions between the 

 chromosome groups of the two sexes was 

 at first interpreted on a 'quantitative' basis, 

 as due to graded amounts of 'sex-chroma- 

 tin.' But because even more species were 

 encountered in which no visible chromo- 

 some difference was detected, the formula- 



tion was changed to include also 'qualita- 

 tive differences in sex-chromatin.' Now, sex 

 is being reinterpreted in terms of genes, 

 with investigation by breeding tests pene- 

 trating to detail far beyond the reach of 

 cytological investigation. The chromosomal 

 differences are now treated as rough guides, 

 and chromosomal determination is l)cing 



resolved into genie determination 



Hence the difference in sex must be put on 

 the same basis as that of any other organ 

 differentiation in plant or animal, for ex- 

 ample, the difference between legs and 

 wings in birds— both modifications of an- 

 cestral limbs." 



In contrast to what some have inter- 

 preted. Bridges was not committed to a 

 specific chromosome balance, as for in- 

 stance the X chromosomes to the A chro- 

 mosomes in Drosophila, for all species. 

 Rather he looked on that particular balance 

 as but one mode of gene association in 

 chromosomes that could bring about dif- 

 ferences in gene action on development 

 which would lead to sexual differentiation 

 in its various forms. In that sense his ideas 

 prepared him for, and were in conformity 

 with, the types of sex differentiation which 

 later became established in such forms as 

 Melandrium and in the silkworm. In his 

 concepts of sex determination and in his 

 active interest in making genetics more spe- 

 cific, there can be little doubt that his 

 theory would include those of the present 

 and would also be welcome as refining and 

 more sharply defining the significance of 

 cytologic and genetic elements important to 

 sex differentiation. 



A. CONCEPT OF SEX DETERMINATION 



As frequently happens, observations are 

 made often before their significance is more 

 than dimly understood. Background infor- 

 mation may be insufficient for the facts to 

 become clear. This was true when a lone 

 chromosome was discovered as a part of 

 the maturation complex in sperm formation 

 of Pyrrhocoris. The association of the pres- 

 ence of this element with the idea of its be- 

 ing the arbitrator between male and female 

 development in certain species was not 

 made until 10 years later. Henking (1891) 

 observed in Pyrrhocoris 11 paired chromo- 



