SECT. 4] HEAT-PRODUCTION OF THE EMBRYO 731 



The difference between the oxygen tension of the blood supplied 

 to the embryonic circulation and the blood coming away from the 

 embryo is therefore 60 — 15, i.e. 45 mm. of oxygen — amply sufficient 

 to allow of a diffusion process^. In just the same way, the difference 

 between the carbon dioxide of the maternal arterial blood and that 

 of the blood coming away from the embryo is 61 — 43, i.e. 18 mm. 

 carbon dioxide, and the same argument holds. The maternal and 

 foetal bloods could not be compared directly in Barcroft differential 

 manometers because they have not the same dissociation curve, 

 so Huggett used a differential tonometer method in which gas 

 mixtures were allowed to come into equilibrium with the maternal 

 and foetal blood, and the tensions then estimated in a Haldane gas 

 analysis apparatus. The relative blood-gas tensions then worked out 

 as follows: 



Maternal arterial blood (oxygen) _ i -g 



Foetal venous blood (oxygen) i -o 



Maternal arterial blood (carbon dioxide) _ 10 

 Foetal venous blood (carbon dioxide) i -2 1 



Maternal venous blood (oxygen) _ i -o 



Foetal arterial blood (oxygen) 1-3 



"The first experiment shows", says Huggett, "that the gradients 

 existing between the foetal venous blood going to the placenta and 

 the maternal arterial blood going to the uterus are adequate for 

 diffusion. The third experiment shows that the maternal venous 

 blood in the uterine vein has a lower oxygen tension than the foetal 

 arterial blood which is not surprising if we remember that the 

 placenta, unlike the lung, absorbs an appreciable quantity of oxy- 

 gen." Huggett also did some experiments in continuation of Zuntz's 

 original ones, in which the foetal blood gave up oxygen to the 

 maternal blood. By asphyxia Huggett found that this can actually 

 take place. The reversal of the gas current through the placenta when 

 the gradient on the maternal side is reversed, though difficult to explain 

 on any secretion theory, would be an obvious corollary of diffusion. 



^ Huggett worked with embryos at term, but Kellogg showed that the difference 

 between O^-content of maternal arterial and foetal venous blood in the dog is much 

 greater early in development than it is later on. The difference is gradually lessened by 

 the rising oxygen-content of the foetal venous blood. This may be due to greater 

 oxygen-carrying capacity or to an increase in the ratio placental area/unit foetal weight. 

 Apparently the placenta gives a low margin of safety, for the pulmonary area of the 

 newborn infant is at least twice the placental area, and the oxygen-content of its blood twice 

 that of the foetus at term. Nor is the blood of the latter more than 63 per cent, saturated 

 with oxygen, although the corresponding maternal figure is 95 per cent. (Eastman). 



