NUTRITION OF THE EMBRYO 1085 



or transudation. But something is contributed by the fcetus in the 

 form of renal, and perhaps of skin, secretions. The fluid is poor in 

 solids. Its maximum content of protein, reached during the first half 

 of pregnancy, is only 0-7 per cent. Later on it diminishes, and at full 

 term is only one-tenth of this amount. The specific gravity is 1006 to 

 1009. Its osmotic concentration, as measured by the depression of the 

 freezing-point, is less than that of the mother's blood-serum. 



The allantois, growing out at the umbilicus, in the manner described, 

 insinuates itself between the true and false amnion, and soon blends 

 with the latter. For a time the secretion of the primitive kidneys 

 continues to be poured into the cavity of the allantois, so that it serves 

 in part as an excretory organ, while in the bird it also performs the 

 function of respiration ; and in the mammal both food and 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 tiie uterine wall and this part of the chorion is now distinguished 

 ad 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 fcetal 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. 246), than the purest blood of the fcetus, 

 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 fcetus of a sheep had 47 volumes 

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



