1378 



PHYSIOLOGY 



villi are immersed in maternal blood, the foetal blood-vessels being 

 separated from the maternal blood by a double layer of epithelium, 

 one layer of which is maternal and the other fetal in origin. Later 

 these cells become reduced to a single layer. 



FIG. 562. Diagram to illustrate the imbedding of the ovum in the decidua. 

 and the first formation of the foetal villi in the form of a syncytial 

 trophoblast (derived from the outer layer of the ovum) which is invading 

 sinus-like blood -spaces in the decidua. (After T. H. BRYCE.) 



NUTRITION OF THE EMBRYO. At the earliest period of its 

 development the fertilised ovum is dependent for its nourishment 

 on the remains of the cells of the discus proliferus adhering to it, or 

 on the fluid of the Fallopian tube in which it is immersed. The first 

 blood-vessels which are formed serve to take up nourishment from 

 the yolk-sac. In man, however, this source of supply is insignificant, 

 and from the second week onwards blood-vessels traversing the 

 chorionic villi come into close relation with the maternal blood, from 

 which, henceforth, the whole growth of the foetus is to be maintained 

 by a special development of these connections in the placenta. 



In the fully formed foetus blood passes from the foetus to the 

 placenta by the umbilical artery, and is returned by the umbilical 

 veins. There is no communication between fostal and maternal cir- 

 culations. The placenta represents the foetal organ for respiration, 

 nutrition, and excretion. Thus the umbilical artery carries to the 

 placenta a dark venous blood, which in this organ loses carbonic acid 

 and takes up oxygen, so that the blood of the umbilical vein is arterial 

 in colour. The oxygen requirements of the foetus are, however, but 



