no 



BIOLOGIC BASIS OF SEX 



Fig. 2.21. A. The normal ovary of a young female aged 30 days; note the highly developed 

 cortex and the absence of conspicuous structures in the medullary area. B. Transformed 

 testis of a young male treated with estradiol dipropionate for a period of 33 days ; for details 

 see text. Observe the remarkable development of the cortex associated, however, with dis- 

 tinct remains of testicular structure in the medulla. 



of females receiving estradiol. Also, preco- 

 cious growth of a certain number of follicles 

 commonly occurs in the cortex of trans- 

 formed testes (Figs. 2.195 and 2.20). This 

 effect, however, may be exerted indirectly 

 in response to gonadotrophic stimulation. 



It should be noted that in the only other 

 case of an embryonic mammalian gonad 

 transformed by hormone action, that of the 

 freemartin, the reversal involves the con- 

 version of ovary to testis. In the opossum 

 the situation is reversed. In those cases of 

 comjjlete, or near complete, transformation 

 in amphibians, in which the sex chromosome 

 complex has been determined, it appears 

 that it is always the homogametic sex that 

 is readily transformed (Gallien, 1955). This 

 generalization would seem to apply also in 

 birds and in the case of the freemartin. The 

 opossum, however, is certainly an exception; 

 the male in this species is heterogametic 

 (Painter, 1922; Tijo and Puck, 1958; Gra- 

 ham, 1956). In certain fishes at least sex 

 genotype is apparently of no conseciuence 

 .-^ince functional sex reversal proceeds 

 eciually well in either direction (Yamamoto, 

 1953, 1958). 



V. The Role of Hormones in the 



Development of the Accessory 



Sex Structures 



The heterogeneous character of the vari- 

 ous structures comprising the genital com- 

 plex of the embryo has been previously 

 emphasized as providing a basis for great 

 variability in their behavior under experi- 

 mental conditions. On the basis of embry- 

 onic origin and morphologic relationships 

 the accessory sex structures fall into three 

 principal groups: (1) the embryonic sex 

 ducts and related structures which are taken 

 over from the primitive nephric system; (2) 

 derivatives of the cloaca or the urinogenital 

 sinus, derived at an early stage from the 

 primitive gut; and (3) external organs of 

 sex. Because of the great diversity of the 

 so-called secondary sex characters in verte- 

 brates, and because as a rule they become 

 sexually differentiated only in jiostpubertal 

 life, these structures can be considered only 

 in special cases. 



Two distinct stages can be recognized in 

 the development of the accessory sex struc- 

 tures: an early phase which is independent 

 of sex, and wliich follows a \-ii-tuallv identi- 



