REPRODUCTION 871 



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, hypertrophy 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 covering of decidual cells, 

 the fcetal vessels are bathed in maternal blood. By this inter- 

 weaving 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. 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 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 (Kuntz and Cohnstein). This, although far from 

 the level of ordinary arterial blood, is yet the best the foetus ever 

 gets ; and by a series of contrivances it is assured that this best 

 should go first to the most important parts, the liver, the heart and 

 the head, while the legs and most of the abdominal organs have to 

 put up with an inferior supply. This is brought about mainly by the 

 existence of three short-cuts for the blood, which disappear in the 

 adult circulation, the ductus venosus, the ductus arteriosus and the 

 foramen ovale (Fig. 336). 



The blood of the umbilical vein, rich in oxygen for foetal blood, 

 passes partly through the circulation of the liver, but a part takes the 



