REPRODUCTION 875 



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 600 small calories. Now, taking the 

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

 mechanical work, we get 1,800 calories per kilo of body-weight in 

 twenty-four hours (see p. 503), or about ^j- of the heat-production 

 of a resting adult. 



So low is the intensity of metabolism in the embryo, so slight the 

 demand for oxygen, that not only is even the purest blood, as has 

 already been stated, far from saturated with that gas, 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 birth. At an early period of development 

 the embryo also contains much more water than the adult; the 

 specific gravity of its tissues increases as development goes on. 



The remarkable vitality of the fcetus, and its resistance to asphyxia, 

 are related to the feebleness of its metabolism and to the compara- 

 tively slight excitability of nervous centres like the respiratory, vaso- 

 motor, and cardio-inhibitory. Even when totally deprived of oxygen, 

 as by pressure on the umbilical cord during delivery, the child does 

 not perish in the two or three minutes which decide the fate of the 

 asphyxiated adult; nor are the convulsions, rise of blood-pressure, 

 and slowing of the heart-beat, associated with asphyxia in the latter, 

 so readily induced, nor premature and fatal efforts at respiration 

 easily excited in utero. But although in such a case the embryo 

 behaves as a separate organism, governed by its own laws, there are 

 circumstances in which it becomes merely a part of the mother and 

 participates in her fate. Thus, the stream of oxygen which normally 

 passes from the maternal to the fcetal blood is turned back if 

 asphyxia threatens the mother ; the blood of the umbilical arteries, 

 instead of being purified in the placenta, loses the little oxygen it 

 holds to the blood of the uterine sinuses, and the sluggish tissues of 

 the embryo are impoverished to feed the more active metabolism of 

 the maternal organs. In the same way, the phenomena of starvation 

 have taught us that the nutrition of the organism is not subject to 

 the rules of red tape. In normal circumstances the flow of nutriment 

 follows definite lines : the blood feeds the tissues through its inter- 

 mediary, the lymph, and recoups itself from the contents of the 



