1126 THE NUTRITION OF THE EMBRYO. [BOOK iv. 



of the gases in the maternal and foetal blood. At least we have 

 no more evidence in the case of this placental respiration than 

 we had in the case of the pulmonary respiration that the inter- 

 change is in any way assisted by cellular activity of a secretory 

 kind. The placental respiration of the mammal seems in fact 

 exactly to repeat the branchial respiration of the fish ; in the 

 former the foetus breathes by means of the maternal blood 

 in the same way that in the latter the fish breathes by means of 

 the water in which it lives. 



It follows from the above that the foetus may be asphyxiated 

 in two ways : on the one hand by interference with the access of 

 foetal blood to the placenta, as when the cord is tied, and on the 

 other hand by the maternal circulation being arrested, or by the 

 maternal blood being wanting in oxygen. When the mother 

 is asphyxiated the foetus is asphyxiated too, the oxygen passing 

 from the foetal blood to that of the mother. In such a case, 

 owing to the more imperious demands of the maternal blood, 

 the store of oxygen in the foetal blood is sooner exhausted 

 and asphyxia is more rapidly developed than in the case when 

 the cause lies in the foetus, not in the mother, and the oxygen 

 simply disappears from the foetal blood as it is slowly used up 

 by the foetal tissues ; for the rate of foetal oxidation though it 

 increases continually during the intra-uterine life, especially in 

 the later stages, is slow compared to what it becomes some 

 time after birth. 



698. The foetus not only breathes but also feeds and 

 probably excretes by means of the placenta ; the blood returning 

 by the umbilical vein is not only richer in oxygen and poorer in 

 carbonic acid but also richer in nutritive material and poorer 

 in waste products than the blood of the umbilical arteries. 

 In dealing however with the nutrition of the embryo we must 

 bear in mind a special condition under which the embryo lives. 

 As we have said the embryo proper becomes at an early date 

 invested with the double membranous bag of the amnion, 

 consisting of the inner amnion and outer (false) amnion. 

 Between the two there is at first a space, into which as we have 

 seen the allantois grows in order to become the placenta; but, as 

 the fluid, which from the first is present within the inner bag, 

 increases in amount, without any corresponding increase in the 

 fluid between the inner and outer bag, the (true) amnion in its 

 expansion after the formation of the placenta reaches and unites 

 with the false amnion which by this time is known as the 

 chorion. The whole interior of the uterus is lined, next to the 

 decidua, by a membrane apparently simple but composed of 

 united amnion and chorion, and within this, surrounding and 

 supporting the embryo, lies the amniotic fluid, which at first 

 scanty rapidly increases in amount until in the later stages of 

 pregnancy it may amount to 800 c.c. or even much more. 



