The chorion is of a very variable thickness. 1st. In the head, that of the cranium and that of the face exhibit an opposite arrangement. The first is very thick and also dense and compact, which is owing especially to the numerous hairs that go through it. The second, everywhere fine and delicate, is particularly so upon the eyelids and the lips. 2d. The chorion of the trunk is posteriorly and all along the back, of a thickness almost double that of its anterior part, where it is nearly the same upon the neck, the chest and the abdomen. I would except however that of the penis, the scrotum, the great labia and the mammæ, in which its delicacy is greater than any where else. 3d. In the superior extremities it is nearly uniform upon the shoulders, the arm and the fore-arm; on the hand it increases a little in thickness and more in the palm than on the back. 4th. This thickness is generally much more evident on the thigh and the leg, where there are more muscles, than on the arm or the fore-arm. On the foot, it increases as on the hand, less in the dorsal than in the plantar region, which is the thickest of all the parts of the dermoid system; which is owing principally in the natural state to the arrangement of its epidermis. We see from this, that though everywhere continuous, the chorion is very different in its different parts. The relation of its thickness with its functions is easily perceived on the hand, the foot, the cranium, &c. Elsewhere we cannot so well see the reason of these differences, which are notwithstanding as constant. Woman has a chorion generally less thick than that of man; compared in all the regions, it exhibits in the two sexes a sensible difference; on the mammæ especially, it is much more delicate in woman. That of the great labia however is proportionally thicker than that of the scrotum. In order to understand perfectly the intimate structure of the chorion, it is necessary to examine it at first on its internal surface, after having carefully separated it from the fatty cellular texture, to which this surface adheres more or less intimately. We see then that it is differently arranged according to the regions.