1012 A MANUAL OF PHYSIOLOGY 



amniotic fluid acts as a water-jacket or cushion, to break tfre force 

 of the inevitable shocks and jars transmitted from the mother 

 to the foetus and from the foetus to the mother. To some extent, 

 in addition, it may serve as a nutritive fluid, for substances can 

 pass from the blood of the mother into the amniotic fluid, and the 

 amniotic fluid can be swallowed by the foetus. This is shown by 

 the fact that sodium sulphindigotate, when injected into the maternal 

 circulation, is found in the amniotic fluid and in the alimentary canal 

 of the foetus, although not in any of the foetal tissues. Fine lanugo 

 hairs from the foetal skin have also been found in the meconium. 



The precise origin and manner of formation of the amniotic fluid 

 have not been settled. It is probably in the main a maternal 

 secretion or transudation. But something is contributed by the 

 foetus 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 primi- 

 tive 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 bypogastric 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 foetal 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. 



