DEVELOPMENT OF ACCESSORY STRUCTURES. 159 



As development advances the decidua reflexa increases in size, and at 

 about the end of the fourth month comes in contact with the decidua 

 vera, with which it is ultimately fused. 



The Placenta. Of all the embryonic structures, the placenta is the 

 most important. It is formed in the third month, and then increases in 

 size until the seventh month, when a retrogressive metamorphosis takes place 

 until its separation during labor, at which time it is of an oval or rounded 

 shape, and measures from seven to nine inches in length, six to eight 

 inches in breadth, and weighs from fifteen to twenty ounces. It is most 

 frequently situated at the upper and posterior part of the inner surface of 

 the uterus. 



The placenta consists of two portions, a foetal and a maternal. 



The Feet al portion is formed by the villi of the chorion, which, by devel- 

 oping, rapidly increase in size and number. They become branched and 

 penetrate the uterine tubules, which enlarge and receive their many ramifi- 

 cations. The capillary blood vessels in the anterior of the villi also en- 

 large and freely anastomose with each other. 



The Maternal portion is formed from that part of the hypertrophied and 

 vascular decidual membrane between the ovum and the uterus, the decidua 

 serotina. As the placenta increases in size, the maternal blood vessels 

 around the tubules become more and more numerous, and gradually fuse 

 together, forming great lakes, which constitute sinuses in the walls of the 

 uterus. 



As the latter period of gestation approaches, the villi extend deeper into 

 the decidua, while the sinuses in the maternal portion become larger and 

 extend further into the chorion. Finally, from excessive development of 

 the blood vessels, the structures between them disappear, and as their walls 

 come in contact, they fuse together, so that, ultimately, the maternal and 

 foetal blood are only separated by a thin layer of a homogeneous substance. 

 When fully formed, the placenta consists principally of blood vessels inter- 

 lacing in every direction. The blood of the mother passes from the ute- 

 rine vessels into the lakes surrounding the villi ; the blood from the child 

 flows from the umbilical arteries into the interior of the villi ; but there is 

 not at any time an intermingling of blood, the two being separated by a 

 delicate membrane formed by a fusion of the walls of the blood vessels 

 and the walls of the villi and uterine sinuses. 



The function of the placenta, besides nutrition, is that of a respiratory 

 organ, permitting the oxygen of the maternal blood to pass by osmosis 

 through the delicate placental membrane into the blood of the foetus ; at 

 the same time permitting the carbonic acid and other waste products, the 



