HORMONES IX DIFFERENTIATION OF SEX 



135 



many types of grafting experiments, and 

 above all by the results of embryonic cas- 

 tration. At the same time, steroid hormones 

 of the adult type are also capable in many 

 cases of reproducing closely the effects of 

 the embryonic hormones, thus suggesting a 

 basic similarity. This is not to say, however, 

 that in all groups and species the role of 

 hormones in the differentiation of sex is 

 the same, either with respect to their effects 

 on individual structures or their part in the 

 differentiation process as a whole. Along 

 with ontogenetic processes in general, hor- 

 monal relationships have evolved differently 

 in the different vertebrate groups. 



Amphibians. In amphibians both sex hor- 

 mones seem to have active and essentially 

 coordinate roles in sexual differentiation. 

 With respect to gonad differentiation, 

 grafted gonads of opposite sex induce trans- 

 formation of both testes and ovaries by 

 acting selectively on the appropriate gonad 

 components. In many cases the interacting 

 gonads are reciprocally modified, both be- 

 coming strongly intersexual ; when this re- 

 ciprocal effect does not occur it is appar- 

 ently due to the decisive predominance of 

 one member. Moreover, the gonad com- 

 ponents in many cases react in a similar or 

 identical manner to both natural and syn- 

 thetic hormones. Male hormones induce 

 precocious differentiation of the male duct 

 system and the cloacal glands in individuals 

 of either sex and when administered early 

 may also completely suppress the develop- 

 ment of the jMiillerian ducts; conversely, 

 female hormones stimulate differentiation 

 of the ]\Iiillerian ducts but are without effect 

 on such male structures as Wolffian ducts 

 and cloacal glands. But if such evidence 

 demonstrates beyond question that the lar- 

 val sex structures are capable of reacting 

 specifically to hormones of the proper type, 

 the role of hormones in the normal differ- 

 entiation of sex is more directly demon- 

 strated by the effects of larval castration. 

 In castrates of either sex the gonaducts and 

 other sex accessories remain indefinitely in 

 an undifferentiated or slightly differentiated 

 condition. The sexually neutral type in am- 

 phibians thus tends to be morphologically 

 intermediate between the sexes; however, 

 either sex type may be readily obtained 



from the castrate type by transplanting an 

 ovary or a testis (p. 112). The positive role 

 of both hormones in sex differentiation is 

 apparent. 



Birds. In bird embryos also steroid sex 

 hormones stimulate precocious growth and 

 differentiation of the appropriate sex pri- 

 mordia, and in general have inhibitory ef- 

 fects on structures of the other sex. There is 

 a high degree of specificity in the interaction 

 between hormone and end organ, and from 

 the results of hormone administration alone 

 it might be inferred that the two hormones 

 have coordinate roles in sex differentiation. 

 However, the results of castration show 

 clearly that the roles of the two hormones 

 are different and unequal. The Miillerian 

 ducts persist and continue to develop in a 

 similar manner in castrates of both sexes 

 (p. 115). The male hormone is evidently 

 the decisive factor in their differentiation 

 since the presence of the testes causes in- 

 volution of the ducts in males whereas the 

 ovaries are not essential for their develop- 

 ment in females. On the other hand the sex- 

 type of the genital tubercle and the syrinx is 

 conditioned by the ovaries. Both structures 

 are normally developed in castrate males, 

 for which male hormone is evidently not 

 essential, whereas in castrate females also 

 they closely approach the male condition 

 in both form and size. It is the inhibitory 

 action of the ovary, therefore, that deter- 

 mines the dimorphism of the syrinx and the 

 genital tubercle. ^^ These conclusions are 

 confirmed by the fact that when isolated 

 and cultured in vitro the Miillerian ducts, 

 and the genital tubercle and syrinx, behave 

 exactly as in castrate embryos; however, 

 addition of male hormone to the culture 

 medium causes involution of the Miillerian 

 ducts, and female hormone prevents male 

 differentiation of the tubercle and syrinx. 



Significant differences thus appear in the 

 reactions of the accessory sex structures in 

 birds as compared with amphibians. In the 



'^This i.s true also of such striking sex charac- 

 ters as phunage type and spurs in adult fowl, 

 whereas the head furnishings are under the con- 

 trol of the male hormone. There is, however, much 

 ■\-ariation in the relationships of secondary sex 

 characters and gonad hormones in birds (Domm, 

 1939). 



