482 



Special Vertebrate Organogenesis 



sion of the prostate may be induced by a 

 single dose of hormone administered just be- 

 fore the prostatic buds should appear (Burns, 

 '42); and prostatic glands induced in female 

 embryos continue to develop without further 

 treatment (see Moore, '47). Thus, bud forma- 

 tion represents a critical stage in which de- 

 velopment or permanent suppression of the 

 prostate is determined by presence of the 

 proper hormone. 



Reactions of the Copulatory Structures to 

 Hormones. In both birds and mammals, hor- 



ferentiation is provided by castration of the 

 embryo. The effects of castration in larval 

 amphibians have been mentioned (p. 478), 

 and successful techniques have recently been 

 developed for birds and mammals (Fig. 189). 

 To be decisive the operation must be per- 

 formed early. After early castration the Miil- 

 lerian ducts of male embryos, instead of 

 retrogressing, continue to develop (as does 

 also the abortive right oviduct in female 

 chicks). Presumably in normal development 

 regression is conditioned by the testis — in 



Fig. 188. The effects of male and female sex hormones on the development of the external genital struc- 

 tures in young opossums, treated from birth to an age of 20 days (from Burns, '49). A, Precocious develop- 

 ment and hypertrophy of the phallus (male type) in a male embryo receiving male hormone; B, genital 

 structures of female type produced in another male embryo by female hormone. When both embryos are 

 female the effects are the same. 



mones control development of the copula- 

 tory structures, producing a complete change 

 in morphology in many species, e.g., the 

 duck (Wolff and Wolff, '48), the opossum, 

 (Moore, '41; Burns, '42, '45b), and various 

 placental mammals (Greene, '42; Raynaud, 

 '42; Jost, '47a; Wells and van Wagenen, '54). 

 The embryological basis for transformation 

 has been studied in the opossum (Burns, 

 '45b). It consists in specific responses of the 

 histological constituents of the embryonic 

 phallus to a given hormone. The erectile 

 tissues, which largely determine the form 

 of the penis, are stimulated by male and 

 virtually suppressed by female hormone, 

 which in turn produces marked hyperplasia 

 of the periurethral and vulvar connective tis- 

 sues (Fig. 188). 



The Effects of Castration or Isolation on the 

 Development of Sex Primordia. The most 

 direct test of the role of hormones in sex dif- 



the case of the right oviduct of the chick by 

 the ovaries (Wolff and Wolff, '51). Is de- 

 velopment of Miillerian ducts after castra- 

 tion due solely to release from inhibition, or 

 does some positive humoral factor intervene? 

 [In various species of mammals estrogens 

 are known to be present in the placenta and 

 fetal fluids. For a review see Price ('47).] 

 This question has been approached by test- 

 ing the developmental capacity of the ducts 

 in isolation. After explantation of the em- 

 bryonic genital tracts in vitro (Jost and 

 Bergerard, '49; Wolff, '50; Jost and Bosic, 

 '51), or transplantation to the chorio-al- 

 lantois (Wolff, '50), the Mullerian ducts of 

 both sexes persist and continue to differ- 

 entiate. When explanted after the eighth day 

 of incubation, however, those of male chicks 

 degenerate (Wolff). At this stage regression 

 has been finally determined. The evidence 

 indicates that in the absence of testes the 



