I002 REPRODUCTION 



full term.* The blood-pressure in the umbilical artery of the 

 mature embryo (sheep) varies from 60 to 80 mm. of mercury; 

 but at the beginning of the aorta it will be more. The pressure in 

 the pulmonary trunk must be about equal to that in the aorta, since 

 the comparatively short and easy circuit through the lungs does 

 not as yet exist; and in accordance with this equality of pressure 

 (of work to be done) is the equality of thickness (of working power) 

 in the walls of the two sides of the heart. 



Suppose, now, that the embryo contains 60 grammes of blood for 

 every kilo of body- weight, and that the whole of the blood passes 

 through the circulation in twenty seconds. Then in twenty-four 

 hours 259-2 kilos of blood will be forced through the heart for every 

 kilo of body- weight against a pressure of, say, 80 mm. of mercury, 

 or i metre of blood. This is equivalent, in round numbers, to 260 

 kilogramme-metres of work, or 0-6 calories. Now, taking the total 

 heat-production of the heart at three times the equivalent of its 

 mechanical work, we get 1-8 calories per kilo of body-weight in 

 twenty-four hours (see p. 663), or about -$ of the heat-production 

 of a resting adult. 



Such movements of the skeletal muscles as occur cannot account 

 for any large proportion of the total metabolism, since they are 

 executed in a medium (the amniotic fluid) of nearly the same specific 

 gravity as that of the body, and therefore require the expenditure of 

 a very limited amount of energy. The ordinary functional activity 

 of the embryo, then, is quite incapable of accounting for the intensity 

 of the foetal metabolic processes. Still less can it be due to an active 

 combustion in the tissues to compensate for a rapid loss of heat, 

 for the foetus lies sheltered in the uterus as in a thermostat at its 

 own temperature, and can lose practically no heat unless its tempera- 

 ture be kept a little above that of the maternal blood. The only 

 remaining explanation of the magnitude of the fcetal metabolism 

 is that the growth processes are associated with a large amount of 

 oxidation (and cleavage). 



Notwithstanding the intensity of metabolism in the embryo, not 

 only is even the purest blood, as has already been stated, far from 

 saturated with oxygen, but the relative proportion of haemoglobin, 

 the oxygen-carrier, is less than in the adult ; and although constantly 

 increasing in amount from the moment of its first appearance, it is 

 still somewhat deficient, even at full term, but leaps sharply up at 



* It has not been finally determined whether the rate of the heart varies 

 with the size or, what probably comes to the same thing, with the sex of the 

 foetus. As we have seen, the variation of the rate in the adult with the size 

 of the body is associated with a corresponding variation in the metabolism 

 and heat-loss, which are proportionally greater in a small than in a large 

 animal. If this is a causal connection we should not expect that in the 

 embryo in utero, where the conditions as regards heat-loss are entirely different, 

 such a relation should exist, at any rate within the same species. 



