Structure of the Villositiee (Figs. 132, 133).—The villosities themselves are composed of a layer of epithelium, of vessels, and of a mucoid con nective tissue. They are simply prolongations of the superficial chorio nal stroma, with its numerous connective tissue cells, and abounding vas cular net-work, which lifts up over itself a layer of epithelium. This latter is cylindrical, and is continuous over the villi and the spaces between them. Branches of the chorional vessels penetrate the villi and break up into plexuses which are known as vascular tufts. This vascular net-work lies immediately under the epithelium. This is the disposition of the vessels in ruminants, according to Dastre; but in the human species it is different. Each villosity has an afferent artery, and an efferent vein, which run side by side to the • extremity of the villus, and anastomose there by means of a series of branches (•obin), or by means of a- little vascular net-work (Schroeder Van der Kolk). The vessels form the axis of the villus; and below the epithelium is a very delicate membrane sup posed by Dastre to be the 'homogeneous membrane which Goodsir and Schroeder Van der Kolk described in the human placenta, and the existence of which is denied by Schenk.
Each cotyledon has an infundibuliform depression at its base; the allan toic layer passes like a bridge from border to border of this cavity, and thus closes the enlarged orifice. The chorion seems as if depressed and pushed back by the development of the axillary vessels at the time when the villosities were formed. Thus the interval between the two mem branes is transformed into a sort of cavity irregularly partitioned by in filtrated mucoid tissue. Thus the fcetal placenta is formed from the fold ings of a membrane analogous to the ordinary mucous membranes, such as that of the intestine. (Dastre.) 2. Uterine Placenta.—The study of the uterine placenta is inseparable from that of the utero-placental mucous membrane. Winkler divides the maternal portiOn of the organ into three parts, a para-uterine part, or basal platte, a sub-chorional part, sehluss platte, and an intermediate part, pars cavernosa. (See Fig. 134.) Leopold has followed month by month the changes that occur in the mucous membrane and in the placenta.
At the end of the fourth month, the fetal placenta forms a compact mass with a prominent border, between which and the decidua vera there is a furrow running all around the organ, out of which the decidua re fiexa, the chorion and the amnion spring up to cover the placental vault. At the placental edge the mucous membrane is .12 inches thick, and closely resembles the uterine mucous membrane. It is divided into a spongy and a compact layer. In the spongy portion are several layers of wide glandular spaces, the outer ones of which still possess a cylindrical epithelium, but the inner ones are mere empty spaces' or contain only epithelial debris. The meshes of the net-work sustain the vessels which
penetrate the thick vascular trunks of the muscular coat. In the com pact layer, on the other hand, sections of small and of medium-sized vessels may be seen at one and the same time. As the glandular spaces become larger and more irregular, we find here and there a border of cuboidal epithelium. In consequence of the pressure and the develop ment of the placenta, the glands are heaped up in the serotina; they become longer and more slender. Finally the glands become separated from the placenta by an edge about .039 of an inch thick, which forms the compact layer. It is composed of greatly enlarged vessels, and of layers of nucleated decidual cells. If we follow the border of this layer.we notice projections from it at distances of .18 to .39 inches, with smaller tuberances between. The first correspond to the partitions of the pla rental cotyledons, and the others form the little depressions at their level. The maternal placenta is thus formed of this compact layer, which con tains only a few glandular spaces; after childbirth it is expelled with the placenta. This layer, therefore, must be separated from the spongy layer by the thickness of the glandular layer, and consequently this last layer must remain attached to the surface of the placenta, together with the debris of the torn glands. This is the layer that we call the mater nal placenta, and which is intimately united to the villosities. (See Fig. 135.) According to Langhaus, the tissue of the decidua spreads and grows all over the tops of the villi, which are plunged deep into the maternal pla centa, between its cells and dilated capillaries, but without reaching the glands. In young placentas the villi can be drawn out from their seats of implantation in the maternal organ; the epithelial layer of the villosi ties remaining visible upon the maternal placenta, like the finger of a glove. Langhaus distinguishes two kinds of villi, in one of which the branches are plunged into pools of blood, while in the other the principal trunk penetrates the maternal placenta, and is fixed in it by a club shaped end.
Between all these villi, large and small, there lie at this period capil laiies and blood-spaces: Part of the villi are solidly wedged into the ma ternal placenta, and part of them penetrate into largely dilated capillaries which are transformed into cavernous sinuses and are situated in the superficial layer of the mucosa. Some are fixed in the maternal tissues while others float freely in blood-spaces.