DEVELOPMENT OF BIKDS AND MAMMALS 253 



by a coagulable fluid. The yolk sac in many mammals is 

 brought into close relationship with the blood system of the 

 uterus, a yolk-sac placenta being formed. With the develop- 

 ment of the allantois and its spread to form the foetal portion 

 of the placenta, the yolk sac becomes less important, and in 

 some mammals, indeed, it never really reaches the serosa. 

 The allantois mesoderm spreads on the inner side of the serosa, 

 and it at once becomes vascular, establishing a network of 



Amniotlc cavity 



Aud.ves. 



Mi 



Lung 

 Bt. 



Foreb. * Thyroid 



Allantois 



Yolk sac 



FIG. 129. Diagrammatic longitudinal section of mammalian'embryo 

 (of about ten days in the case of the rabbit). 



vessels in the folds of the serosa which are imbedded in the 

 uterine wall. The vitelline veins are reinforced by allantoic 

 veins, and allantoic arteries are developed from the dorsal 

 aorta. The placental circulation is thus introduced, and it 

 forms the means of food and oxygen supply for the rest of the 

 uterine life. It is to be noted that the maternal blood and 

 the foetal circulation are independent, but brought into close 

 relationship in the placenta. The wall between them is the 

 serosa, and it is double over the area involved in placentation. 

 This is a fact of importance, for disease germs of the maternal 

 blood are thus normally shut out from the foetal circulation 

 and only active parasites manage rarely to overcome the barrier. 



