THE LUNGS.--The lungs of the Mammalia differ from those of animals lying lower in the series by the fact, just referred to, that they occupy a pleural cavity completely shut off from the abdomen by the diaphragm. As a rule the lungs of the Mammalia are to be distinguished by their more or less extensive lobation. In the Whales, however, and in the Sirenia, they are not much divided, but present the appearance of the simple sac-like lungs of the reptiles. In some mammals there is a median and posterior unpaired lobe of the lung, which lies in the post-pericardial cavity behind the pericardium. This is not universally present. The lungs are very frequently not symmetrical in their lobation, the number of separate lobes on the right side {70} and on the left being different. The lungs of mammals agree with those of the lower reptiles in being freely suspended within their coelomic cavity, and in not being, as in birds, crocodiles, and the Varanidae among lizards, tied down to the dorsal surface of that cavity by a sheet of peritoneum covering them. [Illustration] FIG. 44.--Part of a sagittal section of an ovary of a child just born. _bl.v_, Blood-vessels; _foll_, strings and groups of cells derived from the germinal epithelium becoming developed into follicles; _g.ep_, germinal epithelium; _in_, ingrowing cord of cells from the germinal epithelium; _pr.ov_, primitive ova. (From Hertwig, after Waldeyer.) THE GONADS (OVARIES AND TESTES).--The ovary in the Mammalia is always paired; there is never a partial or complete abortion of one gonad as in birds--except of course in pathological cases. The ovaries are small, and lie in the abdominal cavity behind the kidneys. In the immense majority of the Mammalia the ova which are produced within the ovaries are of minute size; those of even the colossal Rorqual are, so far as we know, not markedly larger than the ova of a Mouse. The smallness of size of these reproductive elements implies necessarily an absence of much nutritive yolk; and as a consequence the developing embryo, since it is not hatched in an early stage as a free living larva, has to be nourished by the mother, to whose tissues it is attached through the intermediary of the placenta, a structure partly composed of foetal structures derived from the embryo, and partly of portions of the lining membranes of the uterus of the mother. The ova of the {71} Eutherian mammals, including the Marsupials, are very small as compared with those of any other vertebrates, excepting only _Amphioxus_, where the young are hatched early as free swimming larvae. They also differ in a highly characteristic way in the mode of their development within the ovary. These processes are to some extent illustrated in Fig. 44. The main framework of the ovary is formed of the so-called "stroma," which is a mass of tissue formed of more or less connective-tissue-like cells. Within this are numerous cavities, the Graafian follicles.