Numerous flat vessels, obviously veins, terminating in minute subdivisions, are seen ramifying over the whole surface, but be coming very scanty, or ceasing pear the cen tral point. They are continuations of similar vessels, which are still more conspicuous upon the parietal decichia. The capillaries in which these vessels terminate are exceed ingly numerous, and may be sometimes seen deeply injected with blood. This is rendered the more conspicuous when the congestion is unequal, so as to form patches of a bright pink, alternating with other portions of a pale flesh colour.
The internal surface of the fcetal chamber, after the ovum has fallen out, or has 'been re moved, presents a slightly uneven appearance, occasioned by numerous very shallow pits or depressions, occurring in close-set groups, and resembling, upon a small scale, the areolm upon the inner surface of the heart.
When the body of the embryo begins to acquire length, the entire ovum exchanges the spherical for the slightly oval form, and to this the fcetal chamber also becomes adapted. Such is found to be the form of the fcetal chamber, sometimes in the latter half of the first, but generally during the second month, and from this period onwards the ovate figure prevails.
In the latter part of the first month, or at latest in the beginning of the second, the ovum, previously lying loose in the fcetal chamber, begins to be attached to the walls which sur round it. This attachment is effected by the extremities of the villi, which from the first equally surround the chorion, everywhere be coming attached to the little pits and anfrac tuosities upon the inner surface of the fcetal chaniber just described. In this way the em bryo, surrounded by its amnion and chorion, becomes securely anchored in the midst of its little chamber, through the instrumentality of the villi, which, spreading in all directions, may be compared to the rays of the geometric spider's web.
Thus to receive, to protect, and support the ovum, and to prevent its escape from the uterus, appears to be the first object of the formation by the reflected decidua of a sepa rate fcetal chamber (fig. 453.).
Ultimately, as the ovum grows, the base of its chamber expands, and here takes place a niore dense and rapid growth of decidua. This is the part commonly termed the decidua serotina. Here the chorion villi, which now form large minified groups, attach themselves, and from the margins of the collections of sulci just described, into which the villi pene trate, and which are now much extended, there proceed offsets or dissepinients of de cidual structure. These dip down between
the groups of villi sometimes as far as the surface of the chorion, and divide that which was formerly one continuous collec tion of ramified chorion fringes, into the separate lobes which characterise the mature placenta.
One or two points remain to be more ex plicitly stated. It may be asked, how does the ovum gain the interior of the fcetal cham ber, or, in other words, how is the decidua reflexa formed around it ? In reply to this, little beyond conjecture can be offered. Of the numerous explanations which have been attempted, few are found to meet all the pe culiarities of the case. It is most probable that either the ovum becomes embedded in some of those folds of decidua which are found in it at an early period of pregnancy, and so the decidua becomes built up around it, as Sharpey and Coste suppose. Or, as it appears to me more likely, the ovum, on first reaching the uterine cavity, drops into one of the orifices leading to the utricular follicles, and in growing there draws around it the already formed, but soft and spongy decidua constituting the walls of the cavity. The chief support for such a conjecture, beyond its apparent probability, is the fact ascertained by Bischoff; who, in one case in the guinea pig, found the ovum in precisely this situation at the bottom of a uterine follicle.* The entrance of the ovum into the decidua being supposed, the rest of the growth of the reflexa is easily followed. The ovum now, in enlarging, raises the walls of the chamber, in which it lies, just as the skin becomes raised by the accumulating contents of a subcuta neous abscess. The process is probably in part purely mechanical, and in part in the nature of an excentric hypertrophic growth ; for the actual substance of the chamber is much increased beyond the material of which it was at first composed. That some of this is borrowed from the parietal decidua, is very probable from the number of orifices of utri cular glands seen upon its surface, which serve to show that the decidua reflexa is so far formed out of pre-existing structures ; but much is also due to the further development of the elemental decidual tissues ; and to the growth of these, the large vascular supply, which the reflexa at first receives, doubtless contributes. The little point, or umbilicus, observed sometimes at the upper pole of the fcetal chamber, may mark the spot at which, upon either of the foregoing hypotheses, the ovum first entered the decidua.