NUTRITION OF THE EMBRYO 1129 



in pa*-t as an excretory organ, while in the bird it also performs the 

 function of respiration ; and in the mammal both food ana oxygen are 

 carried by its vessels to the foetus during the greater part of intra- 

 uterine life. But later on the outgrowth atrophies and disappears, all 

 except its origin from the alimentary canal, which dilates and persists 

 as the urinary bladder, and its bloodvessels, which grow in the form of 

 tufts or loops into the chorionic villi. The vessels are fed by two 

 umbilical arteries which arise from the hypogastric arteries and run out 

 at the umbilicus on the allantois. The blood is returned by an umbilical 

 vein, whose further course we shall have soon to trace. The shrivelled 

 stalk of the allantois, projecting through the umbilicus, takes part, with 

 its bloodvessels, in the formation of the umbilical cord, which contains 

 also the remains of the yolk-sac and is clothed externally by a layer of 

 the amnion. Continuous with the umbilical cord, and stretching from 

 the umbilicus to the urinary bladder, is a portion of the allantois which 

 is represented in extra-uterine life by a thin cord-like structure, the 

 urachus. The vascular tufts of the chorion, which at first cover the 

 whole surface of the ovum and suck up food and oxygen from decidua 

 serotina and reflexa alike, disappear in the region of the reflexa, hyper- 

 trophy all over the serotina that is, where the ovum is in actual contact 

 with the uterine wall and this part of the chorion is now distinguished 

 as the chorion frondosum. The giant villi of the chorion frondosum 

 push their way into the thickened decidua serotina, and ultimately 

 penetrate into the great capillaries or sinuses of the uterine mucous 

 membrane. At the same time the tissue of the villi external to the 

 vessels becomes reduced to a mere film, so that, except for a thin cover- 

 ing of decidual cells, the foetal vessels are bathed in maternal blood. 

 By this interweaving of decidua and chorion frondosum is formed the 

 placenta, which for the rest of intra-uterine life acts as the great 

 respiratory, alimentary, and excretory organ of the foetus. 



Exchange of Materials in the Placenta. The maternal blood, as 

 it streams through the colossal capillaries of the decidua, gives up 

 to the foetal blood oxygen and food substances and receives from it 

 carbon dioxide and in all probability urea. It is true that the blood 

 in the uterine sinuses is not itself fully oxygenated; it is not bright 

 red arterial blood. But it yet contains more oxygen, and oxygen at 

 a higher partial pressure (p. 247 ), than the purest blood of the foetus, 

 and is, therefore, able to part with some of the surplus to the dark 

 stream of oxygen-impoverished blood brought by the umbilical 

 arteries to the placenta. Thus, it has been found that while the 

 blood of the umbilical artery of the foetus of a sheep had 47 volumes 

 per cent, of carbon dioxide, and only 2-3 of oxygen, that of the 

 umbilical veins had 6-3 volumes of oxygen, and only 40-5 of carbon 

 dioxide (Zuntz and Cohnstein). In the exchange of gases between 

 the placental and the foetal blood the same general features present 

 themselves as in the external and internal respiration of the mother, 

 with this difference, that the exchange of oxygen is neither between 

 air and haemoglobin, as in the lungs, nor between haemoglobin and 

 tissue elements, as in the organs; but between maternal and foetal 

 haemoglobin, of course, through the mediation of the maternal and 

 foetal plasma. There is no reason to suppose that the mechanism 



