CORRELATION OH ENDOCRINE GLANDS WITH SEX DIFFERENTIATION 891 



The foregoing examples and many others (Witschi, '39) suggest the fol- 

 lowing interpretations relative to sex determination and differentiation: 



( 1 ) The germ cell, regardless of its genetic constitution, develops into an 

 egg or a sperm, depending upon whether it lies in a developing cortex 

 or in a developing medulla. That is, the influence of the sex field gov- 

 erns the direction of germ-cell differentiation (fig. 22). 



(2) The sex field is a powerful factor in determining sex. A factor (or fac- 

 tors) which enables an elevation to partial or complete dominance on 

 the part of one sex field, which under normal conditions is suppressed, 

 may result in the partial or complete reversal of sex. 



(3) Differentiation of sex is dependent upon an interplay between the genes 

 of the sex chromosomes and the bio-chemical forces present in the 

 gonadal sex field. This interplay may be considered to work as follows: 

 (a) If the male-sex field or medulla in a particular species is stronger 

 than the female field or cortex, that is, if it is able to compete for sub- 

 strate substances more vigorously and successfully and to produce dif- 

 fusible hormonal substance more plentifully, it will suppress the female 

 sex field. Under these conditions, the chromosomal sex-determining 

 mechanism is established in such a way that the male is the hetero- 

 gametic sex, composed of XY or XO chromosomal combinations, 

 and the female is XX, the genes of the extra X chromosome being 

 necessary to override the male tendency present normally in the male 

 sex field, (b) On the other hand, if the female sex field or cortex 

 is stronger physiologically, then the female is the heterogametic sex 

 (XO or ZW), the homozygous condition of the sex chromosomes 

 in the male being necessary to suppress the natural tendencies toward 

 supremacy of the stronger female sex field, (c) It may be that the 

 general characteristics and strength of the sex field are controlled by 

 genes present in certain autosomal chromosomes, whereas the specific 

 role which the particular sex field takes normally in sex differentiation 

 is controlled by the genes in the sex chromosomes. 



2. Influence of Hormones on the Differentiation of Sex 



The possible effects of hormones upon sex differentiation, particularly upon 

 the development of the accessory duels, have been studied with great interest 

 since F. R. Lillie's ('17) description of freemartin development in cattle. He 

 tentatively made the assumption that the male fetal associate of the free- 

 martin produces a hormonal substance which, through the medium of vascular 

 anastomoses within the placentae of the two fetuses, brings about a partial 

 suppression of the developing ovary and effects, in part, a sex reversal in the 

 developing reproductive organs of the female. The female member of this 

 heterosexual relationship, therefore, is more or less changed in the direction 

 of the male; hence, the common name freemartin. 



