THE DECIDUA. 607 



with the known facts in regard to other Mammals, render it 

 practically certain that the view first advanced by Sharpey is 

 correct, and that, immediately after the ovum has attached itself 

 to the uterine wall, the decidua grows up as a fold around and 

 over it, so as to encapsule it ; the object being, partly to maintain 

 the ovum in contact with the uterine wall ; and partly, perhaps 

 mainly, to provide an increased extent of vascular surface from 

 which the embryonic villi can draw nutriment. 



The fold of the decidua which incloses, or encapsules, the 

 ovum is spoken of as the decidua reflexa; it is at first very 

 thin (Fig. 175, DX, and Fig. 254, dn-), but it has the same struc- 

 ture as the other parts of the decidua. In its early stages it is 

 exceedingly vascular, the vessels converging from its margin to 

 a small patch of a cicatricial appearance on its most prominent 

 part, which probably indicates the point of meeting, and fusion 

 of the folds by which it is formed. 



The part of the decidua to which the ovum is directly 

 attached, and from which the decidua reflexa is developed, is 

 called the decidua serotina (Fig. 175, dw, and Fig. 254, ds) ; 

 while the term decidua vera is given to the whole of the rest of 

 the decidua, which lines the cavity of the uterus, but has no 

 direct relation with the embryo (Fig. 175, DV, and Fig. 254, dv). 



The decidua vera plays no pai't in the nourishment of the 

 embryo, and during the latter half of pregnancy becomes greatly 

 reduced in thickness, and undergoes degenerative changes ; that 

 it should be formed at all is due, as already noticed, to the fact 

 that, as it is quite uncertain with which particular part of the 

 uterine wall the ovum will come in contact, all parts must be 

 ready in the first instance to receive it. 



The fact that the decidua vera, though lining the greater 

 part of the uterus, takes no share in the nutrition of the em- 

 bryo, and after attaining a certain stage, first stops, and then 

 undergoes retrograde development, renders the comparison be- 

 tween the menstrual decidua and the decidua of pregnancy a 

 still closer one ; it further helps to render intelligible the not 

 very uncommon cases in which menstruation takes place at least 

 once after conception has occurred ; and also those much rarer 

 cases in which it has been stated to occur regularly throughout 

 the greater part or even the whole of pregnancy. 



The decidua reflexa 'and decidua serotina are at first of very 



