GENERAL EMBRYOLOGY 1635 



Stated. As it is the source from which the maternal part of the 

 placenta is formed, its future intra-uterine histor\' differs materially 

 from that of the decidua vera and decidua capsularis. The changes, 

 therefore, which this membrane undergoes after the fifth month are 

 associated with the full development of the placenta, which see. 

 Thev may, however, be stated here in continuity with the changes 

 undergone by the decidua vera and decidua capsularis. 



(i) The gland-ca^^ties of the stratum spongiosum undergo 

 changes similar to those which affect the corresponding stratum of 

 the decidua vera. (2) In the remainder of the stratum compactum 

 the uterine glands lose their epithelium and disappear. This 

 vestigial stratum acquires great vascularity, and becomes conse- 

 quently increased in thickness. (3) Superficially — that is to say. 

 towards the region of the foetal part of the placenta — it gives rise 

 to the maternal blood-lacunae, into which the maternal arterioles 

 pour their blood. {4) Underneath — that is to say, on the uterine 

 side of this region of maternal blood-lacunae — there is a layer of the 

 vestigial stratum compactum, which is known a4 the basal layer. 

 This layer adjoins the stratum spongiosum, and is very vascular. 

 The decidua basalis will be again referred to in connection with the 

 maternal part of the placeftta. 



The Placenta. 



The placenta is a ver\' vascular organ, which establishes an impor- 

 tant connection between the body of the foetus and the maternal 

 uterus. Its function is respiratory, nutritive, and excretory', during 

 the last six months of intra-uterine life. 



Physical Characters of the Placenta at Full Tenn. — The placenta 

 at full term has the form of a discoid cake, which is spongy in con- 

 sistence. Its average width is about 7 inches, and its thickness at 

 the centre is about i| inches, the circumference, which is the 

 thinnest part, measuring about 23 inches. Its average weight is 

 about 20 ounces. In all these respects, however, it is subject to 

 much variety. It presents two surfaces — namely, foetal and maternal 

 or uterine. The fcetal surface is smooth and concave, and is looselv 

 covered by the amnion, through which the branches of the umbilical 

 arteries can be seen as they di\nde and subdivide before entering 

 the organ. It is connected with the abdominal wall of the foetus at 

 the umbilical orifice by means of the umbilical cord, which is 

 attached to the placenta at, or near, the centre, as a rule. Some- 

 times, however, the mnbilical cord is attached to some part of the 

 margin of the placenta, which then bears a resemblance to a battle- 

 dore. Under these circumstances it is known as a battledore- 

 placenta. 



The maternal or uterine surface is convex, and constitutes the 

 ■vault of the placenta. It is very irregular, being broken up into a 

 number of polygonal masses or lobes, called cotyledons, which are 

 arborescent tufts of chorionic villi. 



