136 



BIOLOGIC BASIS OF SEX 



latter positive stimulation by the proper 

 hormone is necessary to induce final sexual 

 differentiation. In birds, the role of the sex 

 hormones appears to be primarily inhibi- 

 tory; in the absence of gonads Miillerian 

 ducts develop normally in castrates of either 

 sex and genital tubercle and syrinx spon- 

 taneously assume the male form. No posi- 

 tive stimulus is necessary, only release from 

 the inhibitory influence of the opposing 

 gonad. Thus, the castrate type in birds is 

 not undifferentiated or intermediate in type, 

 but is rather a mosaic in which certain char- 

 acters are typically male, others typically 

 female. Again, however, both hormones are 

 essential for the realization of normal sex 

 differentiation but with the female hormone 

 having the major role with respect to the 

 number of structures controlled. 



Mammals. In mammals still another pat- 

 tern appears in the relation of the accessory 

 sex structures to the gonads and their hor- 

 mones. Administration of pure steroid hor- 

 mones demonstrates again that most of the 

 genital primordia, regardless of sex consti- 

 tution, are capable of reacting to the ap- 

 propriate hormone, and if excessive dosages 

 are avoided the responses are in most cases 

 specific. To summarize, administration of 

 male hormone does not significantly affect 

 the development of male embryos except to 

 accelerate the rate of differentiation; in fe- 

 males, on the other hand, it induces differ- 

 entiation of male structures whereas fe- 

 male primordia are inhibited or fail to 

 respond. In like manner, female hormones 

 have a feminizing action on male embryos. 



Again, it might be assumed from the re- 

 sults of hormone administration that the 

 two hormones have comparable roles in nor- 

 mal differentiation. In reality, what is dem- 

 onstrated is the capacities of the sex primor- 

 dia to respond to hormones experimentally 

 introduced; the true role of hormones in 

 normal differentiation is disclosed only when 

 the embryonic genital tract is required to 

 develop in the absence of the gonads or 

 other hormonal influences. Castration of 

 mammalian embryos reveals that normal 

 differentiation depends chiefly if not ex- 

 clusively on the male hormone. Castrated 

 embryos, regardless of genie sex, develo):) 

 female cliaracters (Miillerian derivatives, 



female sinus form and external genitalia, 

 mammary glands) which are almost as well 

 differentiated in castrates as in normal fe- 

 males (Figs. 2.26-2.28, and Table 2.2). Any 

 possible influence of a maternal hormone in 

 this result appears to be excluded (at least 

 for the sex ducts and their derivatives) by 

 studies of development in vitro. Culture of 

 the isolated gonaducts results in persistence 

 and development of the ]\Iiillerian ducts 

 and involution of the Wolffian ducts, re- 

 gardless of the sex of the donor embryo (pp. 

 117, 121) providing always that isolation is 

 carried out before irreversible determination 

 has occurred. 



In mammalian embryos, then, the testes 

 and the male hormone are all imjiortant for 

 the normal differentiation of sex. Moreover, 

 the role of the male hormone is a dual one; 

 its presence is essential to insure retention 

 and development of male parts and at the 

 same time to prevent the differentiation of 

 female structures, which are capable of de- 

 veloping autonomously, regardless of the 

 presence or absence of female hormone. The 

 latter apparently has no essential role in 

 primary sex differentiation. At this point it 

 should be recalled that such a conclusion 

 had in fact been forecast much earlier on 

 the basis of castration of the newborn rat 

 (Wiesner, 1934, 1935; see Burns, 1938b). At 

 birth morphogenesis of the genital structures 

 of young rats is far advanced and profound 

 modifications after castration are not to be 

 expected ; however, it was found that in cas- 

 trate males a marked atrophy promptly ap- 

 peared in such sex accessories as the seminal 

 vesicles and external genitalia, suggesting 

 that a hormonal influence had been re- 

 moved, whereas in castrate females develop- 

 ment proceeded more or less normally until 

 the approach of puberty. These results were 

 confirmed and extended by LaVelle (1951) 

 in the ncnvboi'n hamster. Wiesner (1934, 

 1935) proposed that sex differentiation 

 might be explained on the basis of one hor- 

 mone, the male, the presence or absence of 

 which would account for the two types of 

 development, an hypothesis now confirmed 

 ill a striking way by the results of castra- 

 tion during embryonic and fetal develop- 

 ment. 



The pre-ciiiiiiciit role of the male hormone 



