484 



Special Vertebrate Organogenesis 



emphasized, without positive commitment 

 as to the role of the ovary in female de- 

 velopment. (The suggestion was made that 

 in cases of early transfusion the first effect 

 of the male hormone is to inhibit the ovarian 

 cortex, in effect eliminating the fetal ovary 

 as a factor in later development.) The re- 

 sults of castration in mammalian embryos 

 fit into the above picture in a remarkable 

 way. Absence of the testis and its hormone 

 arrests development of all male characters — 



TESTIS 



Fig. 190. Diagram illustrating possible modes of 

 hormone action on a sex character. Development of 

 a female structure may result from inhibition of the 

 male component of the gonad (A), thus removing 

 an inhibitory influence, as in castration; or after 

 inhibition of the medulla the structure may respond 

 to the differentiating cortex (B). Finally, it may be 

 stimulated directly as in C. A male character may be 

 adversely affected by any of the above modes of 

 action. Whenever the response of a structure ex- 

 ceeds its normal rate of development, direct stimu- 

 lation, as in C, is indicated. 



ducts, sinus derivatives and external genitalia 

 are uniformly affected (see Fig. 189) — while 

 a full complement of female characters ap- 

 pears in castrates of both sexes. It is clear, 

 then, that the male hormone is necessary 

 for development of the male system, and 

 also for inhibition of female development. 

 On the other hand, no essential role for a 

 female hormone is indicated, although the 

 capacity of female primordia to respond to 

 sufficient doses of female hormone is well 

 established (pp. 480-482). The situation con- 

 forms closely with Wiesner's "monhormonic 

 theory," based on the results of castration 

 and hormone treatments in early postnatal 

 life (Wiesner, '34, '35). 



In amphibians and birds, on the contrary, 

 both gonads participate in the differentiation 

 process. In bird embryos, Miillerian ducts 

 persist and develop after castration in both 

 sexes, as in mammals, regression in the male 

 depending on the testis. But differentiation 

 of the genital tubercle (and syrinx) is con- 



ditioned by the ovary, castration producing 

 the male type in both sexes. Administration 

 of hormones produces corresponding effects. 

 Male hormone inhibits Miillerian ducts and 

 produces a genital tubercle of male type, 

 overruling the ovary; female hormone stabi- 

 lizes the Miillerian ducts in the males and 

 inhibits the tubercle. In larval amphibians, 

 of either sex, both gonaducts are retained 

 after castration in a sexually undifferenti- 

 ated condition. Grafting of a gonad then 

 induces development of the proper duct and 

 other sexual characters. Sex-reversed gonads 

 have the same effects. Thus both hormones 

 participate in differentiation. It is difficult 

 to escape the impression that the mam- 

 malian pattern, in which the male hormone 

 is all important, has evolved in adaptation 

 to the conditions of intra-uterine develop- 

 ment. 



Mode of Action of Hormones. In the inter- 

 pretation of hormone action there is always 

 the problem as to how the responses of 

 embryonic primordia are elicited — whether 

 by direct action of a stimulatory or inhibi- 

 tory nature, or indirectly, by depression or 

 elimination of the normal control mecha- 

 nism. The ability of various primordia to 

 develop autonomously after castration indi- 

 cates the importance of the latter alternative. 



The situation in an intact embryo is com- 

 plex. It is possible for an administered hor- 

 mone to act directly on the individual parts 

 of the genital apparatus, or indirectly, by 

 way of the gonad. Direct action on one 

 gonad component may indirectly influence 

 the other, and throvigh it the accessory struc- 

 tures (Fig. 190). In modified gonads his- 

 tological appearances suggest that the ge- 

 netically dominant component is directly 

 inhibited by a heterotypic hormone, allow- 

 ing development of the recessive component 

 (p. 477, see also Willier, '39). This inter- 

 pretation is supported by experiments (p. 

 475) in which removal or depression of the 

 dominant component is followed by recovery 

 and development of the recessive. It is pos- 

 sible, however, that in some cases a hormone 

 may directly stimulate the homotypic gonad 

 component (Vannini, '46). 



In interpreting the actions of hormones 

 on accessory sex structures, the status of the 

 individual primordia must be considered. 

 The results of castration show that some 

 (e.g., sex ducts, female sinus derivatives and 

 genitalia) are capable of a high degree of 

 autonomous differentiation; others (e.g., the 

 prostate) are entirely conditioned by hor- 

 mones. Structures of the first type may de- 



