This progressive divergence of the human from the animal form, which is based on the law of the ontogenetic connection between related forms, is found in the structure of the internal organs as well as in external form. It is also expressed in the construction of the envelopes and appendages that we find surrounding the fœtus externally, and that we will now consider more closely. Two of these appendages—the amnion and the allantois—are only found in the three higher classes of vertebrates, while the third, the yelk-sac, is found in most of the vertebrates. This is a circumstance of great importance, and it gives us valuable data for constructing man’s genealogical tree. As regards the external membrane that encloses the ovum in the mammal womb, we find it just the same in man as in the higher mammals. The ovum is, the reader will remember, first surrounded by the transparent structureless _ovolemma_ or _zona pellucida_ (Figs. 1, 14). But very soon, even in the first week of development, this is replaced by the permanent chorion. This is formed from the external layer of the amnion, the _serolemma,_ or “serous membrane,” the formation of which we shall consider presently; it surrounds the fœtus and its appendages as a broad, completely closed sac; the space between the two, filled with clear watery fluid, is the _serocœlom,_ or interamniotic cavity (“extra-embryonic body-cavity”). But the smooth surface of the sac is quickly covered with numbers of tiny tufts, which are really hollow outgrowths like the fingers of a glove (Figs. 186, 191, 198 _chz_). They ramify and push into the corresponding depressions that are formed by the tubular glands of the mucous membrane of the maternal womb. Thus, the ovum secures its permanent seat (Fig. 186–194). Fig.195. Diagram of the embryonic organs of the mammal (foetal membranes and appendages). Fig. 195—Diagram of the embryonic organs of the mammal (fœtal membranes and appendages). (From _Turner._) _E, M, H_ outer, middle, and inner germ layer of the embryonic shield, which is figured in median longitudinal section, seen from the left. _am_ amnion. _AC_ amniotic cavity, _UV_ yelk-sac or umbilical vesicle, _ALC_ allantois, _al_ pericœlom or serocœlom (inter-amniotic cavity), _ sz_ serolemma (or serous membrane), _pc_ prochorion (with villi).)