In the older and lower groups of the placentals the entire surface of the chorion is covered with a number of short villi; these "chorion-villi" take the form of pit-like depressions of the mucous membrane of the mother, and are easily detached at birth. That happens in most of the ungulata (the sow, camel, mare, etc.), the cetacea, and the prosimiæ; these "mallo-placentalia" (with a _diffuse_ placenta) have been denominated the _indeciduata_. The same formation is present in man and the other placentals in the beginning. It is soon modified, however, as the villi on one part of the chorion are withdrawn; while on the other part they grow proportionately stronger, and unite intimately with the mucous membrane of the womb. It is in consequence of this intimate blending that a portion of the uterus is detached at birth, and carried away with loss of blood. This detachable membrane--the _decidua_--is a characteristic of the higher placentalia, which have, consequently, been grouped under the title of _deciduata_; to that category belong the carnassia, rodentia, simiæ, and man. In the carnassia and some of the ungulata (the elephant, for instance) the placenta takes the form of a girdle, hence they are known as the _zonoplacentalia_; in the rodentia, the insectivora (the mole and the hedge-hog), the apes, and man, it takes the form of a disk. Even ten years ago the majority of embryologists thought that man was distinguished by certain peculiarities in the form of the placenta--namely, by the possession of what is called the _decidua reflexa_, and by a special formation of the umbilical chord which unites the _decidua_ to the foetus. It was supposed that the rest of the placentals, including the apes, were without these special embryonic structures. The _funiculus umbilicalis_ is a smooth, cylindrical cord, from sixteen to twenty-three inches long, and as thick as the little finger. It forms the connecting link between the foetus and the maternal placenta, since it conducts the nutritive vessels from the body of the foetus to the placenta; it comprises, besides, the pedicle of the allantois and the yelk-sac. The yelk-sac in the human case forms the greater portion of the germinal vesicle during the third week of gestation; but it shrivels up afterwards so that it was formerly entirely missed in the mature foetus. Yet it remains all the time in a rudimentary condition, and may be detected even after birth as the little umbilical vesicle. Moreover, even the vesicular structure of the allantois disappears at an early stage in the human case; with a deflection of the amnion, it gives rise to the pedicle. We cannot enter here into a discussion of the complicated anatomical and embryological relations of these structures. I have described and illustrated them in my _Anthropogeny_ (twenty-third chapter).