HORMONES IN DIFFERENTIATION OF SEX 



129 



both sexes (Fig. 2.32). With increasing dos- 

 ages all these effects are accentuated. 



The phallic structures of mammalian em- 

 bryos react to castration according to the 

 pattern already established for the sex ducts 

 and the prostate (Jost, 1947b; Raynaud and 

 Frilley, 1947). In both sexes castration is 

 followed by development of external geni- 

 talia of female type (Fig. 2.28; Table 2.2) ; 

 the male type of differentiation is depend- 

 ent on the testis whereas the female form is 

 capable of developing without hormonal 

 conditioning, in a somatic or asexual man- 

 ner. 



At this point it will be useful to recapitu- 

 late for mammalian embryos the effects of 

 castration, or of early isolation, on the de- 

 velopment of the genital system as a wdiole. 

 It has been shown that in the absence of the 

 gonads, or of any hormonal conditioning, 

 the embryonic sex primordia collectively fol- 

 low the female pattern of development. In 

 all castrates, regardless of sex, the external 

 genitalia and the derivatives of the urino- 

 genital sinus are of female type, the INIiil- 

 lerian ducts persist and continue to develop 

 in a virtually normal fashion, whereas the 

 Wolffian ducts undergo involution. Thus 

 castrates of either sex toward term have fe- 

 male genital systems which are anatomically 

 complete and almost as well developed as in 

 normal females. 



It is noteworthy that the pattern of de- 

 velopment observed in castrate fetuses cor- 

 responds closely with a condition in human 

 subjects known clinically as gonadal dys- 

 genesis. Individuals presenting this anomaly 

 either lack gonads entirely or show evidences 

 of gonadal atresia at an early stage of de- 

 velopment. Regardless of chromosomal sex 

 as established by the Barr test (Barr, 1957) 

 they possess external genitalia of female 

 type and female genital tracts which, how- 

 ever, are of infantile proportions. Recent 

 evidence indicates that some individuals of 

 this type may lack the Y-chromosome, being 

 of XO constitution (Ford, Jones, Polani, 

 de Almeida and Briggs, 1959; chapter by 

 Gowen) . 



In bird embryos the effects of castration 

 on the genital tubercle are similar except 

 that the sex relation observed in mammals is 

 reversed ; in this group the male form of the 



organ corresponds to the asexual condition, 

 which develops without hormonal condition- 

 ing in castrates of both sexes (Fig. 2.25; 

 Wolff and Wolff, 1951). This is not an ex- 

 ceptional finding; it corresponds with the 

 behavior of various other avian sex charac- 

 ters, such as the syrinx {q.v.) , the spurs, and 

 the sex plumage in species such as domestic 

 fowl. The transposed relationship seen here 

 is in line with the dominant role played by 

 the grafted ovary and the greater potency 

 shown by the female hormone in producing 

 sex reversal in the gonads of the chick. 



The developmental behavior of the genital 

 tubercle after isolation in vitro has been 

 studied in the duck, with results which cor- 

 respond with those of castration. Isolated 

 at 7 to 9 clays of incubation, before the be- 

 ginning of sex differentiation in the gonads, 

 primordia of the genital tubercle always as- 

 sume the male form as in castrates, regard- 

 less of the sex of the donor. By the 10th day, 

 however, the sex type has become fixed, and 

 when isolated after this stage differentiation 

 always follows the sex genotype (Wolff and 

 Wolff, 1952b). 



D. DIFFERENTIATION OF OTHER TYPES OF 

 SEX CHARACTER 



Two further examples will be considered 

 as illustrations of the role of hormones in 

 the development of sex characters of quite 

 different type, the mammary glands, and the 

 syrinx of birds. The mammary glands of 

 field mice have been extensively studied by 

 Raynaud (for a summary see Raynaud, 

 1950). The rudiments of the glands first ap- 

 pear as bud-like ingrowths of the epidermal 

 epithelium which penetrate the underlying 

 mesoderm but retain a connection with the 

 epidermis by a constricted neck (Fig. 2.34.4, 

 B). This phase of development follows the 

 same course in both sexes. Toward the 16th 

 day of gestation differences appear in males 

 which coincide with the beginning of mas- 

 culinization of the female genital tract; the 

 mammary buds lose their connection with 

 the epidermis and remain as isolated epithe- 

 lial nodules in the mesenchyme (Fig. 2.34C) . 

 In females, on the contrary, the buds retain 

 their attachment to the epidermis, and as 

 development continues a circular fold ap- 

 pears surrounding the mammary rudiment, 

 which leads to elevation of the nipple. 



