The following investigation was begun three years ago with a view to inquire into the development of the pericardium, but it transpired that this question is inseparable from that of the development of the heart, great veins, and diaphragm. Since the Avian pericardium is complicated by various septa whose exact nature is not generally agreed upon, the embryos of Mammals, especially of Rabbits, have been used for the investigation, and Human embryos have been obtained to illustrate some of the later stages. But, as the embryo of the Rabbit is capricious in its development, it is hardly possible to say upon any particular day after impregnation what stage the embryos may have attained. As a rule, the various members of a litter differ amongst themselves, some being more advanced than others; so that, without this qualification, it would be misleading to specify, as some authors have done, the exact age of the embryo described or depicted. The origin of the Mammalian heart has been elucidated by various observers, especially by Balfour, Hensen His, and Kolliker, and only requires to be mentioned so far as it bears upon the present inquiry. In the Rabbit the organ begins to develop during the first half of the eighth day. And its commencement is indicated by a slight bending of the splanchnopleure into the widely separated halves of the cœlom (fig. 1, lit.). This loop is thicker than the rest of the splanchnopleure, owing to multiplication and elongation of its cells. A t this time the portion of the coelom into which the cardiac loops project is more capacious than the rest, but has not the same lateral extension. Its somatopleure, after running a little way outwards, turns abruptly ventral wards to join the splanchnopleure. At the foremost end of the rudimentary heart, where the bending of the somatopleure is greatest, both membranes disassociate themselves at an early period from the peripheral uncleft mesoblast; but further back they retain their original connexions. The accompanying drawing was made from a section which was slightly oblique, and illustrates both of these points (fig. 1). Afterwards, as development proceeds, the whole of the cardiocephalic region ultimately frees itself in this way from the peripheral uncleft mesoblast.