THE PLACENTA. 



51 



The maternal portio7i of the placenta is contributed by that portion of the. uterine 

 mucous membrane known as the decidua serotina ; its especial peculiarities consist 

 in the intervillous blood-spaces, which may be regarded as derivations from the 

 eroded maternal blood-vessels. As already described, the trophoblast and maternal 

 tissues early come into close relation, and the capillary blood-vessels are opened by 

 the invasion of the foetal tissue, which latter, in turn, is eroded and channelled out 

 by the maternal blood which escapes upon the rupture of the blood-vessels of the 

 mucosa. The extension of the blood-spaces thus originating constitutes the elaborate 

 system of vascular lacunae, or intervillous spaces, forming so conspicuous a part of 

 the fully developed placenta. 



In its earlier changes the decidua serotina closely resemble^ the decidua vera, 

 presenting an inner compact and an outer spongy layer ; by the middle of preg- 

 nancy, however, the previously enlarged glands have entirely disappeared in conse- 

 quence of the atrophy induced by the increasing pressure caused by the augmenting 

 volume of the uterine contents. When the placenta is detached from the uterus the 



Fig. 60. 



Stump of umhilical cord 



Chorion 



Villus 



Placenta 



.'Placental septum 



Decidua serotina 

 »,;. Lme of separation 



.Uterus 



Spiral branches of uterine artery Inner limit Arteries 



of muscle 



Section of placenta and uterus at the seventh month. (Ecker.) 



line of separation passes through the junction of the former spongy and compact 

 layers ; according to Webster, however, the separation occurs in the compact layer. 

 The condensed decidual tissue closing in the vascular lacuna, on the one hand, and 

 covering the surface of separation, on the other, constitutes the basal plate. The 

 latter is continued deeply within the placenta by connective-tissue portions, \.he septa 

 placentce^ which extend between the groups of chorionic villi, forming the cotyledons 

 visible on the outer surface of the placenta as irregular lobules separated by deep 

 furrows. These septa do not reach as far as the chorion except at the margin of the 

 placenta, where they form a thin membranous sheet beneath the chorion, the subcho- 

 rionic occluding plate of Waldeyer. Large, round, multinucleated elements, the 

 giant cells, measuring from .04 to .08 millimetre in diameter, are present within the 

 tissue of the maternal placenta, especially within the basal plate and the septa. At 

 the margin the placental tissue becomes directly continuous with the foetal mem- 

 branes, the chorion and the decidua being closely united. 



The numerous branches of the arteries supplying the uterus pierce the muscular 

 tunic and gain the basal plate : here the arterial vessels lose their muscular coat and 



