PHYSIOLOGY OF THE EMBRYO 319 



of the mother, and the carbon dioxide of the embryo is taken, 

 up by it. 



Metabolism and the corresponding need of oxygen in the 

 embryo is small. The exchange of gases in the placenta is 

 sufficient to maintain the embryo in apnoea. But this con- 

 dition ceases immediately when, by compression of the 

 umbilical cord or by premature rupture of the placenta, the 

 normal exchange of gases in the blood of the embryo is 

 stopped. The blood of the embryo then lacks oxygen, 

 while the carbon dioxide accumulates by which the respira- 

 tory centre is stimulated and premature respiratory move- 

 ments are made. 



The lungs of the embryo are developed from two divertic- 

 uli of the ventral wall of the esophagus and contain no air 

 (atelectatic) ; the alveoli are formed, but are collapsed, i.e. 

 filled by cuboidal epithelial cells. No negative pressure 

 exists in the pleural cavity. When by the first inspira- 

 tory movement after birth air is forced in, the epithelial cells 

 of the alveoli are flattened, the alveoli contain air, and, after 

 some time, negative pressure is developed in the pleural 

 cavity. As to the origin of this negative pressure authors do 

 not agree. 



(c) Nutrition of the embryo. All the nourishment which 

 the embryo needs for its metabolism and growth is derived 

 from the mother organism. In regard to nutrition we can 

 distinguish two periods, one of which corresponds to the 

 vitelline circulation, the other to placental circulation. 

 During the first period the embryo is supplied with food by 

 the blood of the yolk-sac. The food transudes from the 

 vessels of the mucous membrane of the uterus through the 

 mucosa and egg membranes to the yolk-sac. During the 

 placental circulation, however, the embryo takes its food 

 from the blood of the mother present in the placenta. The 

 food transudes from the maternal vessels of the placenta into 

 the foetal placental vessels. As the yolk-sac is of no further 

 importance after the completion of the placental 'Circulation, 

 it gradually 7 diminishes in sixe and finally dwindles away 



