HORMONES IN DIFFERENTIATION OF SEX 



95 



the effect is mediated. Reversal is caused in- 

 directly by a severe inhibition of mesoneph- 

 ric development. Since the sex cords which 

 give rise to the medulla of the testis are de- 

 rived from the mesonephric blastema, in- 

 hibition of this tissue prevents their forma- 

 tion. In the absence of proper medullary 

 development, the cortical rudiment of the 

 testis eventually becomes active to produce 

 an ovary. This, apparently, is another ex- 

 ample of spontaneous differentiation of the 

 heterotypic gonad component when released 

 from domination. 



A summary of the effects of steroid hor- 

 mones in am'phihians. Sex hormones of adult 

 type, such as testosterone and estradiol, 

 have effects which vary greatly in different 

 taxonomic groups of amphibians and also 

 according to experimental conditions, such 

 as dosage, timing, and duration of treat- 

 ment. In many species their effects are spe- 

 cific to a degree, closely simulating the 

 effects expected of natural hormones. Histo- 

 logically complete and in some cases func- 

 tional transformations of the gonads have 

 been produced in a number of species, in- 

 volving two orders and several families 

 (Table 2.1). Nevertheless, in other species 

 only partial or temporary reversals are ob- 

 tained, and negative or even paradoxical re- 

 sults have come from use of the same hor- 

 mones. Constitutional differences between 

 taxonomic units obviously underlie some of 

 the conflicting results. Sex genotype is also 

 involved, because in a particular species 

 reversal may proceed easily in one direction 

 whereas in the other it is difficult or im- 

 possible to produce. Following in part Gal- 

 lien (1955), the results may be tentatively 

 grouped as follows: 



a. In the higher anurans of the family 

 Ranidae, and perhaps also in the Hylidae, 

 the male hormone induces complete, and in 

 many species a permanent reversal of sex. 

 The action of female hormones on the con- 

 trary is highly variable; transformation 

 may be incomplete or unstable, and with 

 high dosages paradoxical or masculinizing 

 effects often appear. On the other hand, loiv 

 doses of the same hormone have in many 

 cases proper feminizing effects in the same 

 species. In one species (Rana catesbiana) 

 complete reversal of sex in both directions 

 has been obtained. 



b. In certain urodeles and lower anurans, 

 female hormones induce a transformation of 

 male gonads which may be complete and 

 stable, as in Pleurodeles and Xenopus, or 

 partial, as in Discoglossus and various spe- 

 cies of Ambystoma. The extent to which 

 partial reversals are attributable to particu- 

 lar experimental conditions is uncertain. 

 Male hormones in general are much less ef- 

 fective, and are prone to induce a paradoxi- 

 cal inhibition or a feminization of the testis. 

 These effects have in some cases been shown 

 to be mediated indirectly, through an in- 

 hibition of nephrogenesis which suppresses 

 the differentiation of the medullary sex 

 cords. The paradoxical effects of female hor- 

 mones, on the other hand, are in many cases 

 a matter of high dosages; how such effects 

 are exerted is unknown but the action is 

 probably indirect. This point will be dis- 

 cussed elsewhere in connection with the 

 problem of paradoxical effects on other sex 

 structures. 



C. SEX REVERSAL IN AVIAN GONADS 



1. Orgariization of Avian Gonads 



In avian as in amphibian gonads a spe- 

 cific morphologic basis for sex reversal ex- 

 ists during early development in the form 

 of medullary and cortical components which 

 have the usual potentialities. In birds, how- 

 ever, the situation is complicated by the 

 peculiar lateral asymmetry which affects in 

 some degree the entire genital system and 

 which is especially pronounced in the fe- 

 male of most species (Fig. 2.12). The sum- 

 mary which follows is based primarily on 

 the chick (for a fuller account see Willier, 

 1939). In the left embryonic ovary the pre- 

 ponderance of the cortex is great, even in 

 the early stages, whereas in the rudimentary 

 right ovary the cortex is essentially absent, 

 being briefly represented by a transient 

 germinal epithelium which disappears even 

 earlier than that of the testis (Wolff, 1948) . 

 In fact, the right ovary virtually ceases to 

 develop at a stage when only medullary 

 tissue (primary sex cords) has been laid 

 down. In the male the asymmetry is mor- 

 phologically less marked but it is expressed 

 nevertheless in the better development and 

 longer survival of the germinal epithelium 

 (potential cortex) on the left testis. These 



