THE PLACENTA. 



61 



of the uterus, until it is forced against the surrounding wall of the uterine cavity, 

 where it fuses with the decidua vera, and thus the cavity of the uterus is 

 obliterated. This fusion takes place towards the end of the second month, and as 

 soon as it has occurred the discoid mass of placental tissue is continuous at its 

 margin with the fused amnion, chorion, and decidua vera (Fig. 78). 



After the second month the foetus lies in the amnion cavity, which is bounded 

 by the fused chorion and uterine wall, except at the lower end of the uterus, where, 

 over the orificium internum, the cavity of the body of the uterus communicates with 

 the cavity of the neck of the uterus ; there the amniotic cavity is bounded by a mem- 

 brane formed by the fused amnion chorion Iseve and the decidua capsularis only. 



And at the end of pregnancy this portion of the membrane is ruptured by the 

 increased pressure of the amnion fluid produced by the contraction of the muscular 

 wall of the uterus (Fig. 88). 



Unchanged part of uterine gland 

 Muscular wall of uterus | ( Maternal vein 



Maternal artery 

 Decidua basilis unchanged part 



Anchoring villus 

 Decidua-stratum spongiosum 

 Unchanged part of uterine gland 



J Maternal blood of intervillous space 

 Trophoblast covering septum of stratum I 

 \ { Intervillous space compactum of decidua 



* Septum of stratum compactum 



ibilical gut 



.Vena unibili- 



calis impar 

 uibilical artery 



Umbilical cord 



Unchanged layer 

 Spongy layer 



Ectoderm of amnion 

 ''used mesoderm of amnion and chorion 



Compact layer Trophoblast of chorion 



FIG. 79. SCHEMA OF STRUCTURE OF COMPLETED PLACENTA. 



Completion of the Placenta. It has already been stated that each secondary villus 

 consists of a vascular mesodermal core covered by a cellular and a plasmodial 

 layer of trophoblast, the latter lying next the maternal blood in the intervillous 

 spaces. As development proceeds and the intervillous spaces become larger, the 

 villi become longer and more complicated, and at the same time the cellular layer 

 of the trophoblast largely disappears, until in the majority of the villi the 

 plasmodial layer alone covers the vascular mesodermal core. 



In still later stages, degenerative changes occur not only in the villi, but 

 also in the chorionic plate of the intervillous spaces and in the basal trophoblast 

 which closes the spaces externally. One of the results of the degenerative pro- 

 cesses is the deposit of fibrinoid material in the place originally occupied by the 

 trophoblast, the object of this process is still unknown ; another is the adhesion 

 of the fibrinous layers on the surfaces of adjacent villi, and the fusion of the 

 villi thus connected into masses of intermingled fibrinous and vascular tissue. 



When the chorionic part of the placenta is completed it consists of (1) the 



