Urinogenital System 



475 



contrast with conditions in amphibians 

 (Humphrey, '28a) and birds (Willier, '39; 

 Wolff and Haffen, '52), in which the indif- 

 ferent gonad primordium in both sexes shows 

 sex-specific organization. 



THE DIFFERENTIATION OF SEX IN 

 THE GONAD 



The lability of gonad organization during 

 the stages when the sex components are be- 

 ing laid down has both a physiological and 

 a structural basis. Development of ovotestes 

 or rudimentary testes from ovaries may be 

 favored by conditions in the graft environ- 

 ment which intei"fere with cortical differen- 

 tiation; but the immediate basis for reversal 

 lies in the presence of a differentiated medul- 

 lary component. Differentiation is essentially 

 a competitive process which may be upset 

 by any condition which effectively inhibits 

 the dominant component — a view which is 

 consistent with the fact that reversal of sex 

 can be induced by various and apparently 

 unrelated experimental procedures. 



Effect of Eliminating the Dominant Gonad 

 Component. The simplest test of this con- 

 cept is direct elimination of the dominant 

 gonad component. In early development this 

 procedure is difficult, but two classic experi- 

 ments may be cited which are based on 

 survival of a recessive component long be- 

 yond the period of sex differentiation. In 

 adult male toads a part of the embryonic 

 cortex survives as the organ of Bidder. After 

 removal of the testes this structure slowly 

 develops into an ovary, capable of producing 

 fertile eggs (Ponse, '24). A parallel case is 

 the removal of the left ovary of the recently 

 hatched female chick. After this operation 

 the rudimentary right gonad, composed 

 chiefly of medullary tissue, frequently de- 

 velops into a testis which may be fertile (see 

 Domm, '39). In each case a long dormant 

 recessive component retains its capacity to 

 produce a gonad of opposite sex when re- 

 leased from inhibition by removal of the 

 functional gonad. 



Sex Reversal Induced by Unspecific or 

 Environmental Agencies. It is well known 

 that in many animals transformation of sex 

 may be induced by various physiological 

 conditions such as nutritional level, the 

 effects of parasitism, disease, or even by fac- 

 tors in the physical environment. A familiar 

 case is the influence of temperature on the 

 differentiation of the gonads in certain 

 amphibians. High temperatures favor dif- 

 ferentiation of the medulla, and ovaries are 

 gradually transformed jntg testes (Witschi, 



'29; Piquet, '30). The primary effect, how- 

 ever, seems to be a degenerative change in 

 the cortex, which is followed by medullary 

 differentiation. Low temperatures, on the 

 other hand, promote cortical development 

 by retarding the differentiation of the me- 

 dulla. Temperature therefore exerts its ef- 

 fects by interference with the dominant 

 component of the gonad, and the result is 

 again consistent with the concept of compet- 

 itive interaction between medulla and cor- 

 tex. A comparable effect of temperature on 

 embryonic mammalian gonads has been 

 shown (Torrey, '50). 



A B 



Fig. 181. Methods of inducing sex reversal in am- 

 phibians by combining whole embryos or gonads 

 of different sex. A, Parabiosis, in which embryos (of 

 the same or of different species) develop with a com- 

 mon circulation; B, orthotopic transplantation, 

 bringing together gonads of different sex in the 

 same individual. 



Hormonal Control of Gonad Differentiation. 



If a local interaction between medulla and 

 cortex is the basic mechanism in the differ- 

 entiation of the gonad,-]- it is also well 

 known that a gonad is able to influence the 

 differentiation of another at a distance (and 

 to control the development of accessory sex 

 structures as well). From this fact arose the 

 theory of hormones as differentiating agents, 

 which has inspired most of the modern work 

 on sex differentiation. The hormone theory 

 of sex differentiation was first developed with 

 special reference to the freemartin problem 

 by Lillie ('16, '17) and Keller and Tandler 

 ('16). The theory has been tested chiefly by 

 two methods: grafting, during the differen- 



f The view that the interactions between medulla 

 and cortex are mediated by special inductor sub 

 stances, of a different chemical order and with a 

 different mode of action from hormones, will be 

 considered later, 



