a uniformly velvety or shaggy appearance, owing to its being covered everywhere with these tufted and compound villosities.

Fig. 197.

The villosities themselves, when examined by the microscope, have an exceedingly well marked and characteristic appearance. (Fig. 197.) They originate from the surface of the chorion by a somewhat narrow stem, and divide into a multitude of secondary and tertiary branches, of varying size and figure; some of them slender and filamentous, others club-shaped, many of them irregularly swollen at various points. All of them termi nate by rounded extremities, giving to the whole tuft a certain resemblance to some varieties of sea-weed. The larger trunks and branches of the villosity are seen to contain numerous minute nuclei, imbedded in a nearly homogeneous, or finely granular substratum. The smaller ones appear, under a low magnifying power, simply granular in texture.

These villi are altogether peculiar in appearance, and quite unlike any other structure which may be met with in the body. Wherever we find, in the uterus, any portion of a membrane having villosities like these, we may be sure that pregnancy has existed; for such villosities can only belong to the chorion, and the chorion itself is a part of the foetus. It is developed, as we have seen, as an outgrowth from the intestinal canal, and can only exist, accordingly, as a portion of the fecundated egg. The presence of portions of a shaggy chorion is therefore as satisfactory proof of the existence of pregnancy, as if we had found the body of the foetus itself.

While the villosities which we have just described are in process of formation, the allantois itself has completed its growth, and has become converted into a permanent chorion. The blood vessels coming from the allantoic arteries accordingly ramify over the chorion, and supply it with a tolerably abundant vascular network. The growth of the foetus, moreover, at this time, has reached such a state of activity, that it requires to be supplied with nourishment by vascular absorption, instead of the slow process of imbibition,

Extremity of VILLOSITY OF CHORION, more highly magnifled; showing the arrangement of

blood vessels in its interior.

The villi of the chorion are therefore very analogous in structure to those of the intestine; and their power of absorption, as in other similar instances, corresponds with the abundance of their ramifications, and the extent of their vascularity.

It must be remembered, also, that these vessels all come from the abdomen of the foetus; and that whatever substances are taken up by them are transported directly to the interior of the embryo, and used for the nourishment of its tissues. The chorion, therefore, as soon as its villi and blood vessels are completely developed, becomes an exceedingly active organ in the nutrition of the foetus; and constitutes, in fact, the only means by which new material can be introduced from without.

The existence of this general vascularity of the chorion affords also, as Coste was the first to point out, a striking indication that this membrane is in reality identical with the allantois of the lower animals. If the reader will turn back to the illustrations of the formation of the amnion and allantois (Chap. IX.), he will see that the first chorion or investing membrane is formed exclusively by the vitelline membrane, which is never vascular and cannot become so by itself, since it has no direct connection with the foetus. The second chorion is formed by the union of the vitelline membrane with the outer lamina of the amniotic fold. Both laminæ of the amniotic fold are at first vascular, since they are portions of the external blastodermic layer, and derive their vessels from the integument of the foetus. But after the outer lamina has become completely separated from the inner, by the disappearance of the

partition which for a time connected the two with each other (Fig. 192, c), this source of vascular supply is cut off; and the second chorion cannot, therefore, remain vascular after that period. But the third or permanent chorion, that is, the allantois, derives its vessels directly from those of the foetus, and retains its connection with them during the whole period of gestation. A chorion, therefore, which is universally and permanently vascular, can be no other than the allantois, converted into an external investing membrane of the egg.

Thirdly, the chorion, which is at one time, as we have seen, everywhere villous and shaggy, becomes afterward partially bald. This change begins to take place about the end of the second month. It commences at a point opposite the situation of the foetus and the insertion of the foetal vessels. The villosities in this region cease growing; and as the entire egg continues to enlarge, the villosities at the point indicated fail to keep pace with its growth, and with the progressive expansion of the chorion. They accordingly become at this part thinner and more scattered, leaving the surface of the chorion comparatively smooth and bald. This baldness increases in extent and becomes more and more complete, spreading and advancing over the adjacent portions of the chorion, until at least two-thirds of its surface have become nearly or quite destitute of villosities.

At the opposite point of the surface of the egg, however, that

Fig. 199.

portion, namely, which corresponds with the insertion of the foetal vessels, the villosities, instead of becoming atrophied, continue to grow; and this portion of the chorion becomes even more shaggy and thickly set than before. The conse. quence is that the chorion afterward presents a very dif ferent appearance at different portions of its surface. (Fig. 199.) The greater part of it is smooth; but a certain portion, constituting about one-third of

the whole, is covered with a soft and spongy mass of long, thicklyset, compound villosities. It is this thickened and shaggy portion,

which is afterward concerned in the formation of the placenta; while the remaining smooth portion continues to be known under the name of the chorion. The placental portion of the chorion becomes distinctly limited and separated from the remainder by about the end of the third month.

The vascularity of the chorion keeps pace, in its different parts respectively, with the atrophy and development of its villosities. As the villosities shrivel and disappear over a part of its extent, the looped capillary vessels, which they at first contained, disappear also; so that the smooth portion of the chorion shows afterward only a few straggling vessels running over its surface, and does not contain any abundant capillary plexus. In the thickened portion, on the other hand, the vessels lengthen and ramify to an extent corresponding with that of the villosities in which they are situated. The allantoic arteries, coming from the abdomen of the foetus, enter the villi, and penetrate through their whole extent; forming, at the placental portion of the chorion, a mass of tufted and ramified vascular loops, while over the rest of the membrane they are merely distributed as a few single and scattered vessels.

The chorion, accordingly, is the external investing membrane of the egg, produced by the consolidation and transformation of the allantois. The placenta, furthermore, so far as it has now been described, is evidently a part of the chorion; that part, namely, which is thickened, shaggy and vascular, while the remainder is comparatively thin, smooth, and membranous.

CHAPTER XI.

DEVELOPMENT OF UTERINE MUCOUS MEMBRANE.— FORMATION OF THE DECIDUA.

IN fish, reptiles, and birds, the egg is either provided with a supply of nutritious material contained within its membranes, or it is so placed, after its discharge from the body of the parent, that it can absorb these materials from without. Thus, in the egg of the bird, the young embryo is supported upon the albuminous matter deposited around the vitellus; while in the frog and fish, moisture, oxygen, saline substances, &c., are freely imbibed from the water in which the egg is placed.

But in the quadrupeds, as well as in the human species, the egg is of minute size, and the quantity of nutritious matter which it contains is sufficient to last ouly for a very short time. Moreover, the development of the foetus takes place altogether within the body of the female, and no supply, therefore, can be obtained directly from the external media. In these instances, accordingly, the mucous membrane of the uterus, which is found to be unusually developed and increased in functional activity during the period of gestation, becomes a source of nutrition for the fecundated egg. The uterine mucous membrane, thus developed and hypertrophied, is known by the name of the Decidua.

It has received this name because, as we shall hereafter see, it becomes exfoliated and thrown off, at the same time that the egg itself is finally discharged.

The mucous membrane of the body of the uterus, in the unimpreg nated condition, is quite thin and delicate, and presents a smooth and slightly vascular internal surface. There is, moreover, no layer of submucous cellular tissue between it and the muscular substance of the uterus; so that the mucous membrane cannot here, as in most other organs, be easily dissected up and separated from the subjacent parts. The structure of the mucous membrane itself, however, is sufficiently well marked and readily distinguishable