The roughness of the dorsal surface of this, the parietal decidua, is occasioned by the membrane having been torn away from its connexion with the muscular coat of the uterus, in the act of abortion. The club-like projections are apparently the bases or blind ends of the hypertrophied utricular glands torn out entire from the substance in which they were previously embedded. When laid open, they are found to contain a small cavity. The cup-like depressions are the halves, or portions of similar, perhaps smaller glands, torn across, so as to leave other portions still attached to the uterus. The meshes are simply the orifices of such glands and of the channels leading to them.
At this and subsequent stages them may be often seen lying within and among these orifices, fine, thread-like ramified filaments, which some physiologists suppose to be utri cular glands, or their epithelial lining, now becoming loosened out and falling away,— a view in which my own observations do not enable me to coincide. Seefig. 451.
As pregnancy advances to the third and fourth months, the uterine chamber expands, the decidua which lines it increases in thick ness in parts to 3-4'", and becomes at the same time more spongy, so that upon section it appears to be composed of flattened spaces or cells, communicating together by wide valvular orifices. These are best seen by examining under water the rough surface of an aborted ovum at that period, or the corresponding portion of the uterus from which it had been torn off. (Fig. 452.) These cells, or areolar spaces, continue to be seen, more or less distinctly, in the decidua throughout pregnancy, but are most cuous near the margins of the placenta.
Surface of the decidua vera more advanced. (After Schroeder van der Kolh.) It is here represented as still attached to the walls of the uterus after the chorion, together with a layer of the decidua, have been peeled off from it. From a uterus at the sixth month of pregnancy, just beyond the margin of the placenta. The orifices and canals are much wider than in the first figure.
They are still divisions of the same ramified canals, or uterine glands, which have been described as found everywhere in the lining membrane of the uterus before impregnation, fig. 438., but now become so dilated and tor tuous as scarcely to be recognisable as the same structures.* In the latter rnonths of pregnancy, the parietal decidua becomes thinner, and loses much of its spongy character, except imme diately around the placenta, where this is still most distinct. It ultimately becomes
blended with the outer surface of the fcetal membranes, and is partly thrown off' with them in the act of birth, while a part remains, form ing a honeycomb layer, attached to the uterine muscular coat.
If next the growth of the decidua reflexa, or decidua ovuli, be traced, this will be found to undergo a development corresponding with that of the ovum, which it encloses and pro tects. The little chamber containing the ovum, which, as already stated, usually occu pies a situation near one of the upper uterine angles (fig.450.), although it may also be found near the lower orifice ( Hunter, " Gravid Uterus," pl. 34., fig. 4.), or elsewhere, appears at first like a small superadded cavity upon the outside of the larger one, or that formed by the parietal decidua. But as the development pro ceeds, the fcetal protrudes gradually into the uterine chamber, in the form of an incomplete sphere, whose upper pole rises free into the uterine cavity, but the lower forms an attached base of greater or less breadth, which is conti nuous in its entire circumference with the parietal decidua. The two chambers are totally distinct, and have no communication with each other. In aborted specimens, an aperture may be sometimes seen in the base or outer surface of the fcetal chamber, or that part which has been torn away from the ute rine substance. In a very early specimen in my possession, and also in another which I have examined, one or more points are dis tinguishable also upon the upper or northern pole of the little spherical chamber, which have the appearance of apertures recently closed. Coste, in his beautiful series of illus trations*, directs attention, in several figures, to a similar spot in the same situation, having the appearance of a recently closed aperture, or umbilicus. These traces of openings in both the upper and lower poles of the sphere, are of consequence, in reference to the expla nation which will be presently offered of the mode of formation of the decidua reflexa and fcetal chamber.
The outer surface of this chamber is nearly bilicus, but become more distinct towards the smooth. TJpon it, however, are seen the orifices of numerous uterine glands. These are usually wanting near the centre, or um circumference, and are very numerous, large, and close set, in the decidual fold at the base, all round the line of apparent reflexion.