CH. LIX.] 



THE DECIDUA 



837 



FIG. 632. Diagram representing the relation of 

 the developing ovum to the decidua at a very 

 early stage. 1, Uterine muscle; 2, epiblast 

 of ovum ; 3, inner cell mass of ovum (hypo- 

 blast) ; 4, decidua basalis ; 5, decidua cap- 

 sularis ; 6, decidua vera ; 7, cavity of uterus. 



which dip into the decidua capsularis and basalis. Inside the chorion 

 is the amnion, a closed sac, which surrounds the embryo and is 

 attached to its ventral wall at the 4. 



umbilicus. The amnion also forms 

 a sheath for the umbilical cord by 

 which the embryo is attached to 

 the inner surface of the chorion; 

 it is filled with fluid, the amniotic 

 fluid, in which the foetus floats; 

 the umbilical cord contains not 

 only the blood-vessels which pass 

 between a specialised portion of the 

 chorion, which forms the foetal 

 part of the placenta, and the em- 

 bryo, but also the remains of the 

 yolk-sac, and the duct by which it 

 is connected with the intestine of 

 the embryo. 



As the ovum grows, the decidua 

 capsularis is expanded over its 

 surface, and as the growth con- 

 tinues the uterine cavity is gradu- 

 ally obliterated, and the decidua capsularis is forced into contact with 



the decidua vera, with which 

 it fuses. 



As the decidua is merely 

 thickened mucous mem- 

 brane, it naturally contains 

 glands which become en- 

 larged as the decidua 

 thickens. It was believed, 

 at one time, that the villi of 

 the chorion entered the 

 glands, but this is now 

 known to be incorrect. The 

 villi enter theinterglandular 

 substance, and, in the human 

 subject, the glands of the 

 decidua capsularis eventu- 

 ally disappear entirely. In 

 the decidua basalis and the 

 decidua vera the superficial 

 portions of the glands also 

 disappear ; their deep portions remain in an almost unchanged condi- 

 tion, and furnish the epithelium for the regeneration of the glands 



Fin. 633. Diagram representing a later stage of develop- 

 ment than that shown in fig. 632. 1, Uterine muscle ; 

 2, villi of chorion of ovum ; 3, coelom ; 4, decidua 

 basalis ; 5, decidua capsularis ; 6, decidua vera ; 7, cavity 

 of uterus ; 8, allantois ; 9, amnion cavity ; 10, primitive 

 intestine ; 11, yolk-sac. 



