486 



It has been shown further that crystalline 

 sex hormones prevent or repair the effects of 

 embryonic castration (e.g., lost, '50; Wells, 

 '50) and, conversely, that embryonic gonads 

 are able to repair castration effects in the 

 adult (Jost and Colonge, '49). Moreover, 

 substances have been obtained from em- 

 bryonic or fetal testes which modify various 

 adult male sex characters (e.g., Leroy, '48; 

 see also Jost, '48). In view of the many other 

 variables which enter into experimental re- 

 sults there seems no necessity at present for 

 assigning embryonic hormones to a different 

 category. There is also no necessity or justi- 

 fication for assuming that they are identical 

 molecularly with any particular steroid hor- 

 mones. 



Embryonic Hormones and Inductor Sub- 

 stances. A similar problem exists with respect 

 to embryonic hormones and the inductor 

 substances (corticin, medullarin) which, as 

 mediators of the interactions between cortex 

 and medulla, have been postulated to control 

 differentiation of the gonads (see, e.g., 

 Witschi, '34, '39, '50). The inductor sub- 

 stances may be characterized as follows: (1) 

 as the primary effectors in sex differentia- 

 tion, they exert their effects early, before 

 embryonic hormones are present; (2) their 

 action is exerted locally, ordinarily within 

 the confines of the gonad; and (3) transport 

 is by diffusion through the tissues rather 

 than by the blood. Thus inductors are hu- 

 moral in nature but not hormones. It is sug- 

 gested further that the great variability in 

 the reactions of gonads under experimental 

 conditions, particularly as regards the many 

 race and species differences, presupposes such 

 specificity in organization as to indicate that 

 the inductor substances are probably protein 

 in nature. 



The theory of inductor substances has be- 

 come difficult to maintain in the face of 

 growing evidence that hormones are capable 

 of producing most of the effects ascribed to 

 the inductors. The mechanism of action in 

 the gonad is apparently the same. When 

 gonads undergo transformations (whether by 

 administered hormones or by gonads acting 

 from a distance, as in parabiosis) the his- 

 tological picture shows a change in the ratio 

 of cortex to medulla. This change is appar- 

 ently produced by inhibition of the heter- 

 otypic gonad component — that is to say, the 

 effect of the hormone is to redirect the mech- 

 anism of normal differentiation. 



Crystalline hormones, male or female, are 

 also capable of controlling the primary dif- 

 ferentiation of gonads, so that all embryos 



Special Vertebrate Organogenesis 



from the beginning develop as one sex (see 

 Padoa, '38, '42; Gallien, '44, '50). The 

 dosages required may be minute (see Mintz, 

 '48). If inductors are present their actions are 

 overruled, the hormone assuming the role of 

 the genetically recessive inductor. It may be 

 urged that hormones work by controlling the 

 inductor systems; but, in the absence of di- 

 rect evidence of the existence of inductor 

 substances, it seems unnecessary to assume 

 two agencies. (For fuller discussions see 

 Willier, '39; Wolff, '47; Jost, '48; Ponse, '49; 

 Burns, '49; Gallien, '50.) The actual evidence 

 seems insufficient to prove that the postu- 

 lated differences between inductor and hor- 

 mone effects are more than quantitative, 

 depending on such factors as intensity (con- 

 centration) and timing, in conjunction with 

 species differences in the sensitivity of re- 

 actor systems. 



Under proper conditions the local action 

 of inductors is closely imitated by hormones. 

 Only a few examples may be cited. In am- 

 phibians (under normal or experimental con- 

 ditions) the first responses of the gonaducts 

 to hormones often take place in close prox- 

 imity to the gonads (normal or grafted); 

 and in chick embryos, gonads and sex ducts 

 both show highly localized reactions to an 

 adjacent graft (Wolff, '46). Finally, the ex- 

 tremely localized action of an embryonic 

 testis in repairing castration atrophy in the 

 adult seminal vesicle (Jost and Colonge, '49) 

 is a case in point. 



REFERENCES 



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