HORMONES IN DIFFERENTIATION OF SEX 



137 



in mammalian sex differentiation stands in 

 contrast with the situation in birds, in which 

 the female hormone has the major role. The 

 parallel with the well known difference in 

 the sex-chromosome complex has been noted 

 but this provides no immediate explanation 

 and may be merely coincidence. On the 

 other hand, another explanation may be 

 found in the special physiologic conditions 

 incidental to the evolution of intra-uterine 

 development in mammals. A situation in 

 which the female hormone has an active 

 role in sex differentiation might present a 

 serious difficulty with male embryos con- 

 stantly exposed during development to the 

 influence of the mother's hormones (the 

 presence of considerable amounts of mater- 

 nal estrogen during pregnancy is an estab- 

 lished fact in many species (Price, 1947; 

 Parkes, 1954). Elimination of the role of 

 the female hormone coincident with the evo- 

 lution of viviparity would then be advan- 

 tageous. The problem of female develop- 

 ment has apparently been met by a change 

 in the status of the female sex primordia 

 which, as amply shown by the results of 

 castration and cultivation in vitro, do not 

 require positive stimulation but develop 

 autonomously unless inhibited l)y the male 

 hormone. 



VIII. The Organization of the Sex 



Priniordiuni and Its Role in the 



Differentiation of Sex 



The role of hormones as specific condi- 

 tioners of sexual differentiation is, however, 

 only one aspect of the problem. Of far 

 greater complexity, and fundamental to the 

 selective character of the differentiation 

 process, are the special attributes of the in- 

 dividual sex primordia which predetermine 

 their reactions to the presence, or absence, 

 of a particular hormone. Each primordium 

 possesses a complex organization, not only 

 as to sex type and morphologic character, 

 but also with respect to such detailed phys- 

 iologic properties as the timing of receptiv- 

 ity, the thresholds at which responses occur, 

 and definite capacities for growth. It is ob- 

 vious that specificity of hormone action does 

 not exist independently of specificity of re- 

 sponse. This organization of the primordium 

 derives ultimately from the genotype of the 



species, operating through the same proc- 

 esses of ontogeny that prescribe the special 

 characteristics of other embryonic parts and 

 systems; it is intrinsic as opposed to the 

 conditioning activities of hormones and 

 other modifying agencies which, with re- 

 spect to the primordium, are external and 

 secondary. Thus, sex primordia may be ex- 

 pected to show variations in behavior to- 

 ward hormones which will be peculiar to 

 and integrated with the patterns of develop- 

 ment characteristic of particular groups or 

 species. 



A. CONSTITUTION AND THE MORPHOLOGIC 

 REPRESENTATION OF SEX PRIMORDIA 



It has been pointed out that even in the 

 bisexual or undifferentiated period of devel- 

 opment many variations are found among 

 different species in the extent to which the 

 structures of the recessive sex are repre- 

 sented morphologically, i.e., are laid down 

 in the form of discrete primordia. The ab- 

 sence or the deficient representation in one 

 sex of certain heterotypic primordia may be 

 normal for particular species, whose pattern 

 of development thus places special limita- 

 tions on sex reversal. In some amphibians 

 reversal is difficult or impossible to in- 

 duce experimentally because of the weak 

 or transient representation of the reces- 

 sive sex component in the gonad; there 

 is no real transformation, merely a severely 

 inhibited, or vestigial gonad. Certain acces- 

 sory structures of the recessive sex may also 

 tend to be abortive or imperfectly devel- 

 oped. This is the case for the Miillerian 

 ducts in the males of various species. In 

 young male opossums, for example, the 

 iNIiillerian duct rarely completes its devel- 

 opment to the point of union with the 

 urinogenital sinus, or the connection if 

 formed is quickly lost, so that the terminal 

 segment of the duct is lacking. Conse- 

 quently, it has never been possible to induce 

 vaginal development in male opossums by 

 treatment with female hormones, although 

 the uterus and Fallopian tube are present 

 and highly developed. Moreover, the tend- 

 ency which leads to absence of this region 

 of the duct in male embryos appears to be 

 present also, but more weakly expressed, in 

 the female. Although the terminal segment 



