I abortion occur at this time, the mucous membrane of the uterus, that is, the decidua vera, is thrown off, and brings with it the egg and the decidua reflexa. On examining the mass so discharged, the egg will be found imbedded in the substance of the decidual membrane. One side of the membrane, where it has been torn away from its attachment to the uterus, is ragged; the other side, corresponding to the cavity of the uterus, is smooth or gently convoluted, and exhibits distinctly the orifices of the uterine tubules; while the egg itself can only be extracted by cutting through the decidual membrane, either from one side or the other, and opening in this way the special cavity in which it is inclosed.

During the formation of the decidua reflexa, the entire egg, as well as the body of the uterus which contains it, has considerably enlarged. That portion of the uterine mucous membrane situated immediately underneath the egg, and to which it first became attached, has also continued to increase in thickness and vascularity. The remainder of the decidua vera, however, ceases to grow as before, and no longer keeps pace with the increasing size of the egg and of the uterus. It is still thick and vascular at the end of the third month; but after that period it becomes comparatively thinner and less glandular, while the activity of growth is concentrated in the egg, and in that portion of the uterine mucous membrane with which it is in immediate contact.

Fig. 268.

Attachment of the Egg to the Uterus.-While the above changes are taking place in the lining membrane of the uterus, the formation of the embryo, and the development of the amnion and chorion have been going on simultaneously; and soon after the entrance of the egg into the uterine cavity, the chorion is everywhere covered with projecting villosities. These villosities insinuate themselves into the uterine tubules, or between the folds of the decidual surface; penetrating in this way into little cavities of the uterine mucous membrane. When the formation of the decidua reflexa is completed, the chorion has already become uniformly shaggy; and its villosities, spreading in all directions from its external surface, penetrate everywhere both into the decidua vera beneath it and into the decidua reflexa with which it is covered. In this way the egg becomes entangled with the decidua, and cannot be readily separated from it without rupturing some of the filaments which have grown from its surface, and have penetrated the substance of the decidua. The nutritious fluids, exuded from the glandular textures of the decidua, are now imbibed by the villosities of the chorion; and a more rapid supply of nourishment

branes.

is thus provided, corresponding in abundance with the greater size of the egg.

Very soon the activity of absorption is still further increased. The chorion becomes vascular, by the formation of blood vessels emerging from the body of the embryo and penetrating everywhere into the villosities with which it is covered. Each villosity then contains a vascular loop, imbedded with itself in the substance of the decidua, and serving to absorb from the uterine mucous membrane the materials for the growth of the embryo.

Subsequently, the vascular tufts of the chorion, which are at first uniformly distributed over its surface, disappear throughout the greater

Fig. 269.

part of its extent, while they become still further developed and concentrated at a particular point, the situation of the future placenta. This is the spot at which the egg is in contact with the decidua. Here, both the decidual membrane and the tufts of the chorion continue to increase in thickness and vascularity; while elsewhere, over the prominent portion of the egg, the chorion not only becomes bare of villosities and comparatively destitute of blood vessels, but the decidua reflexa, which is in contact with it, also loses its activity of growth and

PREGNANT UTERUS; showing becomes expanded into a thin layer, with

out any remaining trace of glandular follicles.

The uterine mucous membrane is therefore developed, during gestation, in such a way as to provide for the nourishment of the embryo in the different stages of its growth. At first, the whole of it is uniformly increased in thickness (decidua vera). Next, a portion of it grows upward around the egg, and covers its projecting surface (decidua reflexa). Afterward, both the decidua reflexa and the greater part of the decidua vera diminish in the activity of their growth, and lose their importance as a means of nourishment for the embryo; while that part which is in contact with the vascular tufts of the chorion continues to grow, becoming excessively developed, and taking part in the formation of the placenta.

CHAPTER XII.

THE PLACENTA.

In all instances in which, as in man and the mammalians, the embryo is dependent, for the materials of its growth, upon nutritious fluids supplied by the uterus, the communication between them is established by means of two vascular membranes. One of these membranes, the chorion or the allantois, is a part of the embryo; the other is the mucous membrane of the uterus. By their more or less intimate juxtaposition, the fluids transuded from the blood vessels of the mother are absorbed by those of the embryo, and thus a transfer of nutriment takes place from the maternal to the foetal organism.

In some species of animals, the connection between the maternal and foetal membranes is a simple one. In the pig, for example, the uterine mucous membrane is everywhere uniformly vascular; its only peculiarity consisting in the presence of transverse folds, which project inward from its surface, like the valvulæ conniventes of the small intestine. The external investing membrane of the egg, or the allantois, is also smooth and uniformly vascular. No special development of tissue or of vessels occurs at any part of these membranes, and no adhesion takes place between them. The vascular allantois of the foetus is simply in close apposition with the vascular mucous membrane of the uterus; each of the two contiguous surfaces following the undulations

Diagram of the FETAL PIG, with its membranes, in the uterus; showing the relation of the allantoic and uterine surfaces.-a, a, b, b. Walls of the uterus. c, c. Cavity of the uterus. d. Amnion. e, e. Allantois.

presented by the other. (Fig. 270.) By this arrangement, transudation and absorption take place from the blood vessels of the mother to those of the fœtus, in sufficient quantity to provide for the nutrition of the latter. When parturition takes place, a moderate contraction of the uterus is sufficient to expel its contents. The egg, displaced from its original position, slides forward over the surface of the uterine mucous

b

membrane, and is discharged without any hemorrhage or laceration of the parts.

In other instances, there is a more intimate connection, at certain points, between the foetal and maternal structures. In the cow, the sheep, and the ruminating animals generally, the external membrane of the egg, beside being everywhere supplied with branching blood vessels, presents, scattered over its surface, a large number of distinct rounded or oval spots, at each of which it is covered with thickly set, tufted, vascular prominences. These spots are called cotyledons, or cups, because each one is surrounded by a raised rim or fold, which embraces a corresponding rounded mass projecting from the internal surface of the uterus. This projecting portion of the uterine mucous membrane is also abundantly supplied with blood vessels; and the tufted vascular loops projecting from the surface of the foetal membrane (Fig. 271, b, b) dip down into its substance and are entangled with those belonging to

COTYLEDON, from the pregnant uterus of the cow.-- a. Internal surface of the allantois. b, b. Fætal blood vessels. c, c. Surface of uterine mucous membrane. d, d. Maternal blood

vessels.

the uterus (d, d). There is no absolute adhesion between the two sets of vessels, but only an interlacement of their ramified extremities; and by careful manipulation the foetal portion, with its villosities, may be extricated from the maternal portion without the laceration of either.

In the carnivorous animals, a similar highly developed, vascular portion of the allantois runs, in the form of a single broad belt or band, round its middle part; and this corresponds in situation with an equally developed zone of the uterine mucous membrane. Here the foetal and maternal structures are adherent to each other; while, elsewhere, toward the two extremities of the egg, they lie simply in contact When gestation comes to an end in these animals, and the foetus, with

its membranes, is expelled, the thickened zone of uterine mucous membrane is detached at the same time, and its place is afterward made good by a new growth.

In the human species, as shown in the preceding chapter, the permanently thickened portions of the chorion and decidua, united with each other by mutual interpenetration and growth, form a single, flattened, cake-like mass of rounded form, occupying rather less than one-third of the surface of the chorion, and corresponding to a similar extent of the inner surface of the uterus. This mass, consisting of the fœtal and maternal elements combined, is the placenta.

The complete development of the placenta takes place in the following manner:

The villi of the chorion, when first formed, penetrate into follicles situated in the substance of the uterine mucous membrane; and after becoming vascular, they are developed into tufted ramifications of blood vessels, each one of which turns upon itself in a loop at the extremity. At the same time the uterine follicle, into which the villus has penetrated, enlarges to a similar extent; sending out branching diverticula, corresponding with the multiplied ramifications of the villus. The growth of the follicle and that of the villus thus go on simultaneously and keep pace with each other; the latter constantly advancing as the cavity of the former enlarges.

But it is not only the uterine follicles which increase in size and in complication of structure at this period. The capillary blood vessels, which lie between them and ramify over their exterior, also become unusually developed. They enlarge and inosculate freely with each. other; so that every uterine follicle is covered with a network of dilated capillaries, derived from the blood vessels of the original decidua.

Fig. 272.

As the formation of the placenta goes on, the anatomical arrangement of the foetal blood vessels remains the same. They continue to form vascular loops, penetrating deeply into the decidual membrane; only they become more elongated, and their ramifications more abundant and tortuous. The maternal capillaries, however, on the outside of the uterine follicles, become considerably altered in their anatomical relations. They enlarge in all directions, and, by encroaching them, fuse successively with each other; and, losing gradually in this

Extremity of a FETAL TUFT, from the human placenta at term, in its recent condition.-a, a. Capillary blood vessels. Magnified 135 diameters.

upon the spaces situated between