Professor v. Baer considers that at a later period the connexion between the decidua and mucous membrane becomes so intimate, that it is impossible to separate the former without also separating the latter from the fibrous tissue of the uterus. This, we apprehend, is the stratum which, as Dr. Hunter observes, "is always left upon the uterus after delivery, most of which dissolves and comes away with the lochia." He does not appear to have been fully aware of the close connexion between the decidua and lining membrane of the uterus, although he evidently observed the fact from the following sentence: "in separating the membranes from the uterus we observe that the adhesion of the decidua to the chorion, and likewise its adhesion to the _muscular fibres of the uterus_, is rather stronger than the adhesion between its external and internal stratum, which, we may presume, is the reason that in labour it so commonly leaves a stratum upon the inside of the uterus." According to the observations of Dr. Montgomery, a great number of small cup-like elevations may be seen upon the external surface of the decidua vera, "having the appearance of little bags, the bottoms of which are attached to, or embedded in, its substance; they then expand or belly out a little, and again grow smaller towards their outer or uterine end, which, in by far the greater number of them, is an open mouth when separated from the uterus: how it may be while they are adherent, I cannot at present say. Some of them which I have found more deeply embedded in the decidua were completely closed sacs. They are best seen about the second or third month, and are not to be found at the advanced periods of gestation."[19] [Illustration: _a_ Uterus. _d_ Decidua reflexa. _b_ Fallopian tube. _e_ Ovum. _c_ Decidua.]