CHAP. XL.] DECIDUA REFLEXA. 579 



called decidua vera. As the ovum increased in size, that part of it 

 towards the Fallopian tube, which of course remained uncovered 

 by the reflexa, at length received a covering, which was termed 

 decidua serotina. 



Decidua Reflexa. Since the time of Dr. W. Hunter, much dif- 

 ference of opinion has arisen among anatomists as to the manner 

 in which the ovum became covered with decidual membrane. That 

 his view was incorrect, follows from the fact which has been ascer- 

 tained by subsequent researches, namely, that the decidua itself 

 consists of the altered mucous membrane of the uterus, and there- 

 fore, like it, is continuous with the mucous lining of the Fallopian 

 tubes, and not stretched across the opening, so as to be pushed 

 forwards by the ovum in its descent. The structure of the decidua 

 reflexa, as was pointed out by Goodsir, differs from that of the 

 vera in not containing many ducts. It does, however, contain a 

 few, which are derived from that part of the decidua vera which 

 is continuous with it. 



Dr. Sharpey considers the central part of the decidua reflexa as 

 formed by a layer of exuded lymph, with which the ovum is 

 covered upon its entrance into the uterus, and that the decidua 

 vera extends upwards around it to a point where these two struc- 

 tures become continuous. He also offers the more simple sugges- 

 tion, that the ovum may first become completely imbedded, and 

 then may carry forward with it a covering of the membrane as it 

 increases in development. A view which agrees in the main with 

 Dr. Sharpey's, and which is probably nearest the truth, is that of 

 M. Coste, who supposes that the ovum becomes partially im- 

 bedded in the decidual membrane and soft pulpy matter poured 

 out by its glands. As the ovum increases in size, the decidua 

 grows up around it, so as ultimately to cover it completely. 



The two layers of decidua above referred to, gradually come in 

 contact as the ovum increases in size, until, towards the end of the 

 third month, they are incorporated together and form one mem- 

 brane, which becomes thinner as pregnancy advances, and is 

 expelled with the foetus at the termination of the period of gest- 

 ation. 



The decidua is a structure admirably adapted for the ovum to 

 be imbedded in in the early stages of its existence, and the 

 numerous glands in its substance seem evidently destined to elabo- 

 rate a pabulum fitted for its nutrition, before its own especial 

 organs are sufficiently advanced in development to select and 

 absorb the materials for its subsistence. 



