8l8 DEVELOPMENT. [CH. LVIII. 



refiexa is formed ; the decidua serotina is that part of the decidua 

 vera which intervenes between the ovum and the uterine wall 

 within the circle of attachment of the decidua reflexa. 



With the subsequent growth of the ovum, the decidua reflexa 

 expands also, encroaching more and more on the uterine cavity, 

 and ultimately coming into contact with the decidua vera, with 

 which it blends, 



The glands of the decidua were at one time supposed to receive 

 the villous outgrowths of the chorion. It has since been shown 

 that these grow into the substance between the glands. The 

 glands, however, furnish a secretion called uterine milk, which 

 assists the nourishment of the embryo previous to the establish- 

 ment of the placental circulation. Later on the glands are 

 obliterated. 



The decidua serotina is the part which undergoes most change ; 

 an irregular spongy tissue is formed in this situation ; and the 

 spaces in the spongework are filled with blood ; the spongework 

 of vascular spaces is incompletely divided into what are called 

 cotyledons by fibrous bands ; and each cotyledon receives a much 

 hypertrophied chorionic villus. It is this conjunction of chorionic 

 villi with decidual tissue that makes up the placenta, which at 

 full term is seven or eight inches across and weighs nearly a 

 pound. 



The placenta is the situation where the foetus receives its 

 nutriment and its oxygen. There is no direct communication 

 between the vascular systems of the mother and foetus. The 

 sinuses of the placenta are filled with maternal blood from the 

 uterine arteries ; the uterine veins carry it away ; but in these 

 blood-spaces the tufts of foetal blood-vessels are hanging. Oxygen 

 and nutriment pass through the walls of the foetal blood-vessels 

 from the maternal to the foetal blood, and carbonic acid, urea 

 and other waste products pass in the contrary direction. The 

 foetal blood leaves the foetus by the two umbilical arteries, which 

 are the terminal branches of the foetal aorta ; these pass in the 

 stalk of the allantois to the placenta, and after undergoing 

 oxygenation in the placental tufts the blood returns by the 

 umbilical vein to the foetus once more. 



Development of the Fcetal Membranes. 



The Yolk-sac. We have already considered the way in 

 which the body of the embryo is pinched off from the yolk-sac. 

 Numerous blood-vessels are developed in its wall, and the 



