546 GENETICS IN RELATION TO AGRICULTURE 



intersexes of Lymantria according to Goldschmidt with the one exception 

 that the quantitative differences between the male and female factors of 

 the female zygote necessary for the differentiation of female characters, 

 are reduced in the free-martin by internal secretions instead of by varia- 

 tions of potency of the male factors in different varieties as in the inter- 

 sexual hybrids of Lymantria." In attributing the free-martin condition 

 to the male hormones Lillie means only to assert that they are the primary 

 causes, and not that they are the decisive factors in each member of the 

 series of events which result in the intersexual condition. He can, how- 

 ever, state confidently on the basis of present results that sex-determi- 

 nation in mammals is not irreversible predestination, and that with 

 known methods and principles of physiology we can investigate the 

 possible range of reversibility. 



It will be observed that neither of these cases invalidates the funda- 

 mental hypothesis that the sex-chromosomes are the normal differen- 

 tiators in sex-determination. Moreover, the sex-chromosome hypothesis 

 has this virtue, that it is based upon observed and firmly established 

 differences between the sexes. It is disappointing in that it provides 

 so little hope for control of the process, but our dissatisfaction with it 

 from this standpoint should not close our minds to its superiority in 

 defmiteness and experimental evidence to all other theories of sex- 

 determination. 



Inheritance of Unusual Sex-ratios. From time to time reports are 

 made of families both in man and other animals which appear to exhibit 

 consistently abnormal sex-ratios. Families are reported in which male 

 children only have been born for a number of generations, or in which 

 only females have been born. Now according to the laws of chance such 

 instances may occur occasionally without necessitating in any way the 

 adoption of hypotheses subsidiary to that of the existence of a mechanism 

 which potentially is calculated to give an approximate equality of the 

 two sexes. But sometimes other factors do appear to be at work, and 

 these may be mentioned briefly here. 



The existence of sex-linked lethal factors in Drosophila has already 

 been pointed out. Presumably these are factors which affect adversely 

 the development or operation of some vital organ as a consequence of 

 which individuals possessing the factor are doomed from the moment of 

 conception to death at some stage in their life history. In some cases 

 this occurs relatively late in the life history. Thus Bridges reports the 

 discovery of a strain of flies with such a factor in which the morescent 

 larvae are distinguished by the production of black specks within the 

 body cavity. These larvae die when they reach maturity, but in other 

 cases death must occur soon after fertilization. Moreover, in some cases 

 the doomed individuals may occasionally overcome the defect and 

 develop into normal adults. 



