PREGNANCY 73 



canal, lung, and kidney for the embryo or foetus, but there is 

 no actual mixing of foetal and maternal blood, the various sub- 

 stances transfusing by osmosis from the blood spaces of the 

 mother into the vessels of the chorion, the two systems of circula- 

 tion being separated by only a thin cellular partition. 



The blood supply to the chorion is brought by the umbilical 

 arteries which grow out from the iliac arteries of the foetus, and 

 the blood is carried back from the chorion to the foetus by the 



Trophoblast 



Mesoderm 



Branches of umbilical vessels 



Fig. 54. — Section across a chorionic vilhis into which mesoderm has 

 grown (secondary villus). (From (Jrai/'s Anatomi/.) 



umbilical vein which passes into the foetal liver. These blood 

 vessels develop from the mesoblastic part of the umbilical cord. 

 It is believed that by the time the foetal blood vessels have formed, 

 the different cells in the body have acquired the power of selecting 

 nutriment of the requisite kind from what is supplied by the 

 blood. At an earlier stage, however, according to Lochhead, the 

 trophoblast probably performs selective and metabolic functions, 

 and so regulates the supply of nutriment in the interests of the 

 developing embryo. Thus it has been shown that the tropho- 

 blast cells are a temporary storehouse for glycogen, fat, and 

 haemoglobin before these substances pass into the foetal circulation. 

 The avidity of these cells for special substances has been demon- 

 strated also by intm-vitam staining, for pyrrhol blue, and other 



