126 Discussion 



placenta rose progressively during pregnancy. Now, blood pressure 

 also rises during the latter half of pregnancy, and it would seem that 

 this rise in blood pressure is in fact due to the development within the 

 foetus of vascular reflexes and the sympathetic nervous system. I 

 wonder, therefore, whether we should not concentrate attention, in 

 future work, on trying to find out what happens to the vascular re- 

 sistance on the foetal side of the placenta. So far as the little evidence 

 available suggests, we may find that the vascular resistance is the same 

 throughout the latter part of pregnancy, and the increase in umbilical 

 blood flow may be due to an increased cardiac output and to changes 

 in the foetus rather than the placenta. 



In any discussion of the transfer of material across the placenta, 

 there are obviously three variables we have to take into account, first, 

 the foetal placental flow, secondly the diffusion coefficient of the pla- 

 cental tissues, and thirdly uterine blood flow. Now, as to uterine blood 

 flow there is practically no data available. Barcroft made some ob- 

 servations in rabbits but the numbers of rabbits were rather small. 

 I know that there is great technical difficulty in measuring maternal 

 blood flow through the placenta but it looks, from Barcroft's other work, 

 as if it might be the limiting factor in asphyxia of the foetus towards 

 the end of foetal life. This seems one of the most important variables 

 which still have to be measured in determining what has been called the 

 lack of efficiency of the placenta. I am not sure that this lack of 

 efficiency has been firmly established, but in order to investigate it, 

 uterine flow must be measured. 



Amoroso : Were you comparing the sheep with the rabbit in respect of 

 maternal flow? 



Dawes: No, the only published work is on the rabbit. 



Amoroso: I raised that point because we might get into difficulties if 

 we try to relate results derived from the rabbit to sheep and vice versa. 

 I think that point was raised, or at least was hinted at by Professor 

 Wislocki when he referred to the maturation of the foetus. The sheep 

 foetus is, as we all know, a very much more mature organism at term 

 than is the rabbit. 



Zuckerman: Prof. Huggett's last remarks about progesterone remind 

 me that McKeown and I once did some work, which was concerned 

 with the effect of progesterone on foetal growth, in which the corpora 

 lutea were destroyed one by one with an electrocautery. We found that 

 as the amount of luteal tissue in the ovaries decreased, so the litter size 

 declined. We, therefore, assumed that progesterone had some direct 

 influence on the foetus. It now seems that Prof. Huggett has demon- 

 strated that the influence of progesterone is on the placenta, and the 

 foetus is affected secondarily. 



Huggett: It is a possible inference. 



Jost: I am not sure that during pregnancy progesterone is necessary 

 for the foetus itself. Two kinds of observations may be mentioned. 



First the experiment of Courrier who succeeded in producing ab- 

 dominal pregnancies in castrated rabbits by opening the uterus at the 

 level of a nidation, and castrating the female on the same day. The 



