28 Discussion 



if you take the average of a number of any age there is a very definite 

 trend; that is to say, the highest values for glycogen in the placenta are 

 achieved round about the 21st day in the rabbit, and there are very few 

 high values late in pregnancy and in the earliest state, about the 15th 

 day, it begins to appear. 



Price: I should like to ask Prof. Jost a question relative to the 

 McManus-positive cells which he demonstrated in the pituitary. The 

 material is very abundant at a certain time and disappears relatively 

 rapidly at a critical period when you say testicular hormone is needed. 

 Do you think there is a special release mechanism which is causing 

 the discharge from the pituitary at that time, or do you think that the 

 pituitary reaches a certain stage and then releases automatically? 



Jost: I cannot explain exactly what happens. I observed changes in 

 the pituitary gland which are concomitant with the need of pituitary 

 hormone for the testis. One answer to your question may be the fact 

 that the same changes occur in the pituitary gland of the female and 

 of the male foetus. The ovary does not seem to need gonadotrophic 

 hormone. The pituitary gland could perhaps function in a self-differen- 

 tiating manner. 



Zuckerman: Prof. Jost, in elaborating your thesis about turning- 

 points in the "determination" of tissues that are conditioned by pitui- 

 tary hormones, it occurred to me that you might be able to alter their 

 time relations by giving gonadotrophin before the critical days on which 

 the pituitary hormones act (22 to 24). 



Supposing you injected gonadotrophic hormone into a foetus on day 

 18 or day 16, would you not accelerate the stage at which masculine 

 organogenesis would proceed normally in the absence of the pituitary? 



Jost: I believe that it might be difficult to expect some action of 

 hormones given too early to the foetus. As far as I know, experiments 

 done with testosterone, for instance in the opossum or in other animals, 

 showed that the prostate area is able to respond only at the time at 

 which normally the prostatic buds appear in the male. So trying to 

 activate this part of the sinus beforehand was not a success. 



Zuckerman: Yes, but I was thinking particularly of the pituitary. 

 Why do you not give gonadotrophic hormone on day 20 or 18, and then 

 move these critical two days forwards? This would surely be the test 

 of your hypothesis. 



Villee: In any such system you have two parts — whatever is producing 

 the hormone, the pituitary for example, and also the cells which are to 

 be stimulated. And Prof. Jost's point, which I think is quite good, is 

 that probably the ability of the cells to respond is the limiting factor 

 here. If you did Prof. Zuckerman's experiment and got a negative 

 result, it still would not disprove Prof. Jost's thesis. There are many 

 examples from the field of amphibian embryology in which there are 

 critical times for response, and you can do what you will before that 

 time and nothing happens. 



Jost: The phase of sensitivity exists in the receptor and you must 

 respect it. 



Zuckerman: Accepting all this, I am still wondering why in pursuing 



