X232 PHYSIOLOGY 



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 foetal and maternal circulations. The placenta 

 represents the foetal organ for respiration, nutrition, and excretion. Thus 



FIG. 564. 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 tropJioblast (derived 

 from the outer layer of the ovum) which is invading sinus-like blood-spaces in the 

 decidua. (After T. H. BRYCE.) 



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 small. It is protected from all loss of heat, movements are 

 sluggish or for the most part absent, and the only oxidative processes are 

 those required in the building up of the developing tissues. On the other 

 hand, the foetus has need of a rich supply of food-stuffs, which it must 

 obtain through the placental circulation. It is imagined that the epithelium 

 covering the villi serves as an organ for passing on the necessary food-stuffs 

 from the maternal to the festal blood in the form best adapted for the 

 requirements of the foetus. We know, however, practically nothing as to the 

 changes or mechanism involved in this transference. Although most of the 

 organs of the foetus are fully formed some time before birth, they are for the 

 most part in a state of suspended activity. The nitrogenous excreta are 



