OIL LIX.] 



THE FCETAL APPENDAGES 



839 



decidua basalis which produces the placenta, which, at full term, is 

 seven or eight inches across, and weighs nearly a pound. 



The placenta is the organ of foetal nutrition and excretion. Its 

 blood sinuses are filled with maternal blood, which is carried to them 

 by the uterine arteries and away from them by the uterine veins. 

 Into these blood-filled spaces he vascular foetal villi project; hence it 

 is easy for exchanges to take place between the fcetal and the 

 maternal blood, though the two blood-streams never mix together. 

 Oxygen and nutriment pass from the maternal blood through the 

 coverings of the fcetal vessels into the fcetal blood, and carbonic acid, 

 urea, and other waste pro- 

 ducts pass in the contrary 

 direction. The foetal blood 

 is carried to the placenta by 

 the umbilical arteries, which 

 are the terminal branches 

 of the aorta of the foetus ; 

 these pass to the placenta 

 by the umbilical cord, and 

 the blood is returned, 

 through the cord, by the 

 umbilical vein. 



Development of the 

 Foetal Appendages and 

 Membranes. 



The manner in which 

 the primitive intestinal 

 canal is separated from the 

 yolk-sac during the folding 

 off of the embryo from the 

 ovum, has already been con- 

 sidered (p. 834). 



In birds the yolk-sac affords nutriment till the end of incubation, 

 and the omphalo-mesenteric blood-vessels which convey the nutriment 

 to the embryo, are correspondingly well developed. In mammalia, 

 the office of the umbilical vesicle ceases at an early period, for the 

 quantity of yolk is small, and the embryo soon becomes independent 

 of it, on account of the intimate relations established with the 

 maternal blood in the placenta. In birds, moreover, as the yolk-sac 

 empties, it is gradually withdrawn into the abdomen of the chick 

 through the umbilical opening which then closes over it. In mammals 

 it remains outside the embryo, and in man its remnants, in a con- 

 tracted and shrivelled condition, are found in the umbilical cord. In 

 some mammals, however, it plays a much more important part than it 



FIG. 635. Diagram representing a later stage of develop- 

 ment of membranes and placenta than that shown in 

 fig. 683. 1, Uterine muscle ; 2, placenta ; 3, yolk-sac ; 

 4, fused decidua vera and capsularis ; 5, primitive blood- 

 vessel of embryo ; 6, amnion cavity (outer surface of 

 amnion is fused with inner surface of chorion) ; 7, um- 

 bilical cord ; 8, foetal villus in placenta. For blood- 

 vessels see subsequent figures. 



