848 THE NUTRITION OF THE EMBRYO. 



V 



bryo may for some time draw the material for their continued construction 

 at first hand from the yolk-sac or umbilical vesicle, both this and they con- 

 tinue probably until the allantois is formed to receive fresh material from 

 the mother by direct diffusion. 



790. As the thin-walled allantoic vessels come into closer and fuller 

 connection with the maternal uterine sinuses, until at last in the fully 

 formed placenta the former are freely bathed in the blood streaming 

 through the latter, the nutrition of the embryo becomes more and more 

 confined to this special channel. The blood of the foetus flowing along 

 the umbilical arteries effects exchanges with the venous blood of the 

 mother, and leaves the placenta by the umbilical vein richer in oxygen 

 and nutritive material and poorer in carbonic acid and excretory prod- 

 ucts than when it issued from the foetus. 



As far as the gain of oxygen and the loss of carbonic acid are concerned 

 these are the results of simple diffusion. Venous blood, as we have already 

 seen, always contains a quantity of oxy-hsemoglobin,and the quantity of this 

 substance present in the blood of the'uterine veins is sufficient to supply all 

 the oxygen that the embryo needs ; the blood of the foetus, containing less 

 oxygen than even the venous blood of the mother, will take up a certain 

 though small quantity. The foetal blood travelling in the umbilical artery 

 must, in proportion to the extent of the nutritive changes going on in the 

 embryo, possess a higher carbonic tension than that in the umbilical vein or 

 uterine sinus ; and by diffusion gets rid of this surplus during its stay in 

 the placenta. The blood in the umbilical arteries and veins is, therefore, 

 relatively speaking, venous and arterial respectively, though the small ex- 

 cess of oxy-hsemoglobin in the blood of the umbilical vein is insufficient to 

 give it a distinctly arterial color, or to distinguish it as sharply from the 

 more venous blood of the umbilical artery, as in ordinary arterial from or- 

 dinary venous blood. Thus, the foetus breathes by means of the maternal 

 blood, in the same way that a fish breathes by means of the water in which 

 it dwells. 



The blood of the foetus is very poor in haemoglobin, corresponding to its 

 low oxygen consumption. When the mother is asphyxiated the foetus is 

 asphyxiated too, the oxygen of the latter passing back again into the blood 

 of the former ; and the asphyxia thus produced in the foetus is much more 

 rapid than that which results when the oxygen is used up by the tissues of 

 the foetus alone, as when the umbilicus is ligatured and the foetus not allowed 

 to breathe. 



If oxygen and carbonic acid thus pass by diffusion to and from the 

 mother and the foetus, one might fairly expect that diffusible salts, proteids, 

 and carbohydrates would be conveyed to the latter, and diffusible excretions 

 carried away to the former, in the same way ; and if fats can pass directly 

 into the portal blood during ordinary digestion, there can be no reason for 

 doubting that this class of food-stuffs also would find its way to the foetus 

 through the placental structures. We do know from experiment that dif- 

 fusible substances will pass both from the mother to the foetus, and from 

 the foetus to the mother ; but we have no definite knowledge as to the exact 

 form and manner in which, during normal intra-uterine life, nutritive mate- 

 rials are conveyed to or excretions conveyed from the growing young. The 

 placenta is remarkable for the great development of cellular structures, 

 apparently of an epithelial nature, on the border-land between maternal and 

 foetal elements ; and it has been suggested that these form a temporary di- 

 gestive and secretory (excretory) organ. But we have no exact knowledge 

 of what actually does take place in these structures. From the cotyledons 

 of ruminants may be obtained a white creamy-looking fluid, which from 



