Age Factor in Some Prenatal Endocrine Events 19 



palates only if given before day sixteen (this effect of cortisone 

 was discovered on mice by Fraser and Fainstat, 1951). 



In such cases the response of the foetal organism to the same 

 treatment is strictly correlated with its developmental age. 

 In the normal developing foetus, variations in the response of 

 the same target organ to the same endocrine gland may also 

 occur according to the age. The efficiency of the endocrine 

 factor then becomes maximal at certain stages, and it seems 

 not unlikely that the hormonal release might also reach a 

 maximum at the same stages. Some facts presented in this 

 paper will show to what extent such speculations may be 

 suggested by experimental data ; but no effort will be made to 

 give a complete review of the field. 



Testicular function and sexual structures 



The testis is responsible for the development of the mas- 

 culine sexual structures, as was observed in castration experi- 

 ments on the rabbit foetus (Jost, 1947, 1953a); the importance 

 of the age factor in these experiments will be recalled in con- 

 nection with two structures. 



The Wolffian, or mesonephric, ducts exist in both sexes at 

 early stages; in females they persist until day twenty-four 

 when they begin to regress ; in males they develop as deferent 

 ducts and seminal vesicles under the stimulation of the testis. 

 They regress in males also, if castration is performed before 

 day twenty-two ; after castrating on day twenty-three only a 

 small seminal vesicle develops, and after castrating on day 

 twenty-four the Wolffian ducts persist and differentiate along 

 their whole length. They have been "stabilized" for the re- 

 mainder of their life ; from transitory urinary ducts they have 

 developed into definitive male genital ducts. Under the in- 

 fluence of the testis, something happens between about days 

 twenty-two and twenty-four which changes the characteristics 

 of the ducts and which enables them to differentiate as deferent 

 duct and seminal vesicle, even in the absence of the testis. 

 However, at that stage and still for a while, these ducts cannot 



