HORMONES IN DIFFERENTIATION OF SEX 



127 



C. EXTERNAL GENITAL STRUCTURES 



Copulatory organs homologous with those 

 of higher vertebrates are not found in am- 

 phibians. They are developed to an extent, 

 however, in certain birds and reptiles and 

 become highly specialized in mammals. The 

 copulatory organ in amniote embryos devel- 

 ops from a simple primordium, the genital 

 tubercle, which is common to both sexes. It 

 becomes specially developed as the penis in 

 the male but in females it persists in a more 

 or less rudimentary form, known in mam- 

 mals as the clitoris. The genital tubercle of 

 birds arises as a small, conical protuberance 

 just within the cloacal orifice. In chickens it 

 is not highly developed, although larger in 

 the male than in the female, but in the males 

 of ducks, geese, and certain other birds it be- 

 comes considerably larger and more modi- 

 fied, constituting a penis (Fig. 2.25). In the 

 embryos of mammals (except the Mono- 

 tremes) the genital tubercle is external in 

 position, arising as an eminence near the 

 ventral rim of the urinogenital meatus. 



The developing copulatory organs of birds 

 and mammals react readily to sex hormones 

 and are extremely sensitive to castration. In 

 birds the clearest experimental results have 

 been obtained in duck embryos, because of 

 the more pronounced sexual dimorphism in 

 this species. Treatment with female hormone 

 (estradiol benzoatej before the 12th day of 

 incubation completely arrests development 

 of the penis in males, and the rudimentary 

 clitoris of the female may be even smaller 

 than normal (Wolff, Em., 1950 ) ; beyond this 

 age, however, the hormone is no longer ef- 

 fective, the form of the prospective penis 

 having been finally determined. The effects 

 of the male hormone are less precise and it 

 is not essential for normal development (see 

 the effects of castration below) . Testosterone 

 proprionate produces great hypertrophy but 



the structure is not entirely normal, the 

 characteristic spiral form of the penis being 

 imperfectly developed (Wolff, Em., 1950). 

 This abnormality is perhaps a result of over- 

 dosage as the dosages used were undoubt- 

 edly very large. Although in the chick the 

 dimorphism of the genital tubercle is less 

 pronounced than in the duck, it reacts in the 

 same way ; male hormones stimulate and fe- 

 male hormones inhibit growth and morpho- 

 logic differentiation (Reinbold, 1951). 



In mammalian embryos the sex type of 

 the developing genital tubercle, or phallus, 

 is easily controlled by sex hormones (Table 

 2.3) ; in fact, this structure is unusually sus- 

 ceptible to modification and may be com- 

 pletely transformed. Typical are the results 

 in the rat (Greene, 1942), the mouse (Ray- 

 naud, 1942; Kerkhof, 1952), the hamster 

 (Bruner and Witschi, 1946) and in pouch 

 young of the opossum (Burns, 1939a, b; 

 Moore, 1941). The rat and the mouse are 

 similar in their behavior. Male hormone does 

 not affect the development of the penis in 

 males except to produce hypertrophy, but in 

 females the genital tubercle is greatly en- 

 larged and assumes the character of a pe- 

 nis.^' Female hormone has opposite effects; 

 females differentiate normally but in males 

 the tubercle fails to enlarge and a hypo- 

 spadic condition frequently appears. 



In young opossums the form of the copu- 

 latory structures is completely controlled in 

 accordance with the type of hormone given, 

 with results which are identical in the two 

 sexes except for a difference in size (Fig. 

 2.32). The basis for the transformation of 

 the phallus has been analyzed histologically 

 (Burns, 1945b). The various histologic con- 



^' Strangely enough this is not tyi^ically the case 

 in freemartins. The chtoris as a rule is not greatly 

 modified (Lillie, 1917). There are, however, some 

 striking exceptions (Buyse, 1936; Numan, 1843, 

 illustrated in Lillie, 1917, Fig. 29). 



the scrotal sac in males and the presence of the pouch folds and mammary rudiments in 

 females. B. The typical form produced by male hormone (left) and female hormone in 

 specimens treated from birth to an age of 20 days; the normal condition at this age is 

 shown above. This striking difference in form is produced without regard to the sex of the 

 subjects, which in this case are both male (note the scrotal sacs). C. The result of administer- 

 ing male hormone (testosterone propionate) from birth to an age of 50 days, in a female 

 subject (left; note the pouch) and a male littermate. Observe the identity in form but dis- 

 tinctly greater size of the penis in the male. D. Comparison of the effects of estradiol dipro- 

 pionate, given from birth to an age of 30 days, in a female subject (left) and a male litter- 

 mate. In both C and D it is shown that the hormones produce genitalia of typical male or 

 female form, regardless of the sex of the subject. 



