480 



Special Vertebrate Organogenesis 



in its entirety has been questioned (Moore, 

 '44, '47). It is known, however, that some 

 of the most obvious paradoxical effects either 

 are exerted indirectly, or depend on spe- 

 cial experimental conditions. This subject 

 is better discussed after some of the evi- 

 dence has been presented. 



The Effects of Hormones on the Sex Ducts. 

 Although the early differentiation of the 

 gonaducts is independent of sex, they are 



regression (such as normally occurs in fe- 

 males) is initiated (e.g., Raynaud, '42; 

 Greene, '42). Paradoxically, it sometimes 

 causes retention of the ducts in females 

 (Greene). 



Male hormone induces retention and pre- 

 cocious hypertrophy of the Wolffian ducts in 

 both sexes. In mammalian embryos the epi- 

 didymis also hypertrophies, and epididy- 

 mides and seminal vesicles develop in fe- 



VAS DEFERENS 



MESONEPHROS 



- BULBAR GLAND 

 PHALLUS 



POUCH YOUNG - ± 10 DAYS 



VAGINAL CANAL 



Fig. 186. The normal differentiation of the genital tracts in pouch young of the opossum (from Burns, 

 '49). A, Sexually undifferentiated embryo of 10 days, showing male and female sex ducts, urinogenital sinus 

 and phallus; B, sexually differentiated genital tracts in young at 35 days, female characterized by vaginal 

 canals and absence of prostate, male showing absence of vaginal structures and numerous prostatic glands. 



capable of responding early to adequate 

 doses of sex hormones. Female hormone in- 

 duces hypertrophy of Miillerian ducts in fe- 

 male embryos, and retention with hyper- 

 trophy in males. The effects in bird embryos 

 are particularly striking (Fig. 185). In males 

 both oviducts persist and hypertrophy, as 

 does also the right duct of females. However, 

 the period of reactivity is limited; there is a 

 "critical period" for effective action. Re- 

 tention and development of the ducts in male 

 embryos is secured by injections of female 

 hormone up to the tenth day ("stabilization 

 effect"); later treatment is useless (Wolff, 

 '38). Female hormone typically has no ef- 

 fect on Wolffian ducts, but in some mammals 



males (e.g., Greene, '42; Burns, '42; Ray- 

 naud, '42; Wells and van Wagenen, '54). The 

 effects of male hormones on the Miillerian 

 ducts are more complex, depending especially 

 on timing and dosage, and on species differ- 

 ences. In larval amphibians and in chick 

 embryos, treatment during the formative 

 period largely or entirely suppresses the 

 ducts (Fig. 185). [See, e.g.. Burns ('39), 

 Foote ('41), Hanaoka ('41) for amphibians; 

 Wolff ('38, '50), Gaarenstroom ('39), Stoll 

 ('48), Huijbers ('51) for the chick.] This 

 effect is also produced by grafts of the em- 

 bryonic testis (Wolff, '46; Huijbers, '51). 

 For complete suppression the hormone must 

 act early — in chick embryos before the sixth 



