102 



BIOLOGIC BASIS OF SEX 



Primary sex cords 



Rete cords 



Germinal epithelium 



A 



Germinal epithelium 



Medullary cords 



(primary sex cords) 



Cortical cords ^Germinal epithelium 



(secondary sex cords) 



Fig. 2.16. Diagrams illustrating schematically the main features of gonad differentiation 

 in amniote embryos with reference to the origin of the medullary and cortical components. 

 A. Origin of the primary sex cords (medullary cords) from the germinal epithelium. B. 

 Gonad at the indifferent stage of sexual differentiation ; the well developed primary sex cords 

 i-epresent the male or medullary component, whereas the germinal epithelium represents, 

 potentially, the cortical component. C. Differentiation of a testis consists in the further 

 development of the primary sex cords, and the reduction of the germinal epithelium to a 

 thin, serous membrane, accompanied by development of the tunica albuginea. D. Differen- 

 tiation of an ovary consists in reduction of the primary sex cords to medullary cords of the 

 ovary, whereas the cortex is formed by continued development of cortical cords from the 

 germinal epithelium. 



poneiit is often weakly represented (Fig. 

 2.16). In the ovary, medullary cords repre- 

 senting the male component are present but 

 tend to become vestigial in most species.^ 

 In the embryonic testis the germinal epi- 



'^ Notable exceptions should bo mentioned in the 

 case of certain species such as the mole (Godet, 

 1950) and the desman or "water shrew" (Peyre, 

 1955) in which, as a normal condition, the ovarian 

 medulla is so strongly developed as to resemble 

 a testis, and so active physiologically as to pro- 

 duce strong masculinization of many parts of the 

 genital tract. A somewhat similar development 

 and hypertrophy of the medullary component also 

 occurs in the fetal ovary of the horse (for the lit- 

 erature on this unusual condition see Cole, Hart, 

 Lyons and Catchpole, 1933; Parkes, 1954). 



thclium usually disapi)ears early, and with 

 its involution the potentiality for cortical 

 development is permanently lost. On the 

 other hand, hermaphroditismus verus not in- 

 fi'ec|iu>ntly occurs as a developmental anom- 

 aly iu many mammalian species (including 

 man), indicating the existence of a basic 

 bipotentiality. As an example, an extensive 

 literature dealing with this subject in ro- 

 dents has recently been summarized by Hol- 

 lander, Gowen and Stadler (1956) and Kirk- 

 man (1958). Also it must be remembered 

 that the classsical example of an embryonic 

 gonad transformed by the action of a sex 

 hormone is found in the freemartin. In some 



