The phenomena which ensue immediately npon the arrival of tile ovum within the ute rine cavity are, in the human subject, as 3-et unknown. Direct observation of the earliest stages are still wanting, and, unfortunately, the difference between these first steps in the manunalia (except Quadrumana) and man is so considerable, that only a limited aid can be derived from comparative observation. The ovum, when first found in the human uterus, is lodged in a small closed cavity, forming a continuous structure with the decidua which lines the rest of the uterine walls. In this little chamber, which may be formed at any part, but is most frequently seen near one or other of the tuba! orifices, the little spherical ovum lies loose and unattached. In various examples which have been preserved and , figured by different authors of the decidua at this stage, the size of this chamber varies from that of a pea to a hazel nut, and this size it acquires in the second week.
The walls of the cavity containing the ovum, and those forming the lining membrane of the uterus, are nearly alike in appearance and texture. They both consist of decidua, the former constituting the decidua reflexa, the latter the decidua vera of Dr. W. Hunter. For greater distinctness, those names are sometimes exchanged for decidua chorii or ovu/i, and decidua uteri. The latter, accord ing to a suggestion of Dr. M. Baillie, is also occasionally termed parietal decidua.
At this time all the uterine tissues have begun to expand and grow, and the uterine cavity-, the walls of w hich were previously nearly in contact, to enlarge after the manner which in pathology constitutes eccentric hy pertrophy. But, according to the fbregoing description, this cavity now no longer forms one, but two compartments, the one partly inclosed within the other.
Of these two chambers, the newly formed and smaller one contains and supports the ovum, and subsequently the fcetus. It may therefore be termed the fcetal chamber ; the other constitutes the original cavity of the uterus, and may be distinguished as the ute rine chamber ; according to the views of Breschet, it is the hydroperionic cavity. As the fcetal chamber enlarges, and the decidua reflexa becomes more expanded in consequence of the growth of the contained ovuni, it gra dually encroaches upon and finally obliterates the uterine chamber, which can no longer be distinguished as a separate cavity after the fifth or sixth month of gestation.
It will be necessary to examine separately the general characters of these two decidual coats. That which lines the uterine cavity may be first noticed. The parietal decidua, at the very earliest period of pregnancy in which it can be examined, forms a soft and spongy layer, 1 in thickness. That surface hich looks towards the nterine cavity is ele vated into numerous projections, vvhich may be roughly compared to the cerebral convolu tions, though relatively much flatter and less regular than these; between them are nume rous little furrows or channels. The whole surface, both in the sulci and elevations, is covered by numerous minute perforations, corresponding with those formerly described as the orifices of the uterine glands in the unimpregnated uterus. But these orifices, from being enlarged, may now be easily dis tinguished by the unaided eye. They give to the surface a fine eribriform aspect. All these characters are more or less observable also in the decidua lining the uterus, in cases of extra-uterine (tubal) gestation. Along the marginal lines formed by the angles of the cavity, where the decidua is always thinnest, these apertures are large and expanded, but in the elevated spots they are often closed, apparently from lateral pressure, occasioned by the rapid growth of structure.
When early abortion takes place, the whole lining of the uterus, including the decidua reflexa, is often thrown off entire, forming a cast of the uterine cavity. If this occurs in the first fortnight of gestation, the mass re tains the triangular form of the uterus. In each of the three angles is generally found an aperture corresponding with the points at which the membrane had been torn olf from its continuity with the lining of the Fallopian tubes and cervix uteri.
The outer, or dorsal surface of the sub stance expelled, is always rugged. It exhibits numerous little papillary or club-shaped ele vations, and between these much smaller cup like or conical depressions, which are seen by transmitted light to lead, where the membrane is thinnest, directly into the apertures ob servable on the inner surface. At the thin nest points of all, these apertures are so wide, and the cup-like depressions so shallow, that the part has the appearance of a net, the meshes of which still consist of the enlarged orifices of the utricular glands. Hence the epithet " lace-like," often applied to the de cidua in this condition.