DEVELOPMENT AND NOURISHMENT OF YOUNG. The most distinctive character of the mammalia is their of development and of nourishment during the earliest period of life. Excepting only the lowest forms ( Prototheria), all are brought into the world alive, not merely by the retention and hatching of the egg within the oviduct, lout by the ionisation of a new conneetion between the embryo and its mother. The minute ovum ENIIIRYOLOGY, U MAN ) on reaching the interns (or womb) connects itself by a set of root-like tufts of vessels with the maternal ves sels in the wall of the uterus. These tufts form an organ called 'placenta,' by which the embryo absorbs nutriment from the mother's blood, and which conveys back to tire mother's circulation for excretion by her the waste products of meta bolism.
All mammals nourish their young (which at first arc unable to digest any other sort of food) upon milk (q.v.), a richly nutritious fluid se creted by the mother's body in 'mammary' glands, whieh become greatly developed in the female during the periods of gestation and lactation: and, as this is found in no other class, it is tire character by which the entire group is inmost posi tively defined, and from which it derives its name. The mammary glands exist in both sexes. hint, except in very rare cases, it is only in the female that they secrete milk. Their Moldier is less than two, and, when more, is generally nearly proportional to that of the young prodneeil at each birth. That there is direct connection bewcen the number of young and the number of teats has recently been shown in the case of ewes which have been bred to throw' two, three, or more young at a birth. In such cases the number of teats has increased proportionately. Their position varies, being determined in each case by s convenience of approach by the suckling young. In the whales, where prolonged sucking would be difficult on account of their aquatic life, the ducts of the glands are dilated to form a reser• voir from which the milk is ejected into the mouth of the young by a compressor muscle of the mother. In the Prototheria (q.v.) the mam
mary glands have no teats, but the milk simply oozes out. through numerous pores, In the marsupials the teats are well developed, but the glands are provided with special muscles by 1 which milk is forced into the mouth of the ex treniely rudimentary young in the mammary pouch. The skin in the mammalia produces a covering of hair (q.v.). which is a peculiar char acteristic of this group, so that it would he a sufficient definition of a mammal to say that it is an animal producing hair. In some, how ever, it is modified into bristles, scales, and other unusual forms, or it may be almost wholly ab sent, as in whales.
The general structure is typified by that of man, and is abundantly described elsewhere, so that no more need be said here than a few words in reference to the cranium. Among the most ehara eteristie points in the ma align skull it may be mentioned generally (1) that the lower jaw articulates directly with the skull, there being no intervening tympanic bone, such as oc curs in the other vertebrates; and (2) that the occipital bone of the skull articulates with the first vertebra by two condyles, one on ether side of the occipital foramen, as in amphibians, in stead of by a single eondyle, as in the reptiles and birds. in proportion as a mammal is re mote in relationship from man. we find that the cranium is diminished: that the face is prolonged by extension of the jaws and nasal cavities; that the orbits are directed outward, and are less distinct from the temporal fossm; and that the occipital foramen and eondyles are placed toward the posterior part of the skull, instead of occupy ing tire middle of its inferior surface, as in man. For further information as to anatomy and physiology, see the articles SKELETON; TEETH ; and those upon the various organs and functions of the human body, with the articles on com parative anatomy accompanying them; also articles upon the various groups of mammals, as CARNIVORA ; RUMINANT; ELEPHANT: and the like.