possession (79, note). It is evident that under such negative circumstances their view of the structure of the human placenta, and its connection with the uterus, will be repudiated. 90. The circulation of the blood in the Ovum is independent of that of the mother (personal experiment (59), and all the more recent physiologists). The embryo creates its own blood, and through it, sustains its own existence. But its blood, like that of all other animals, whether during its intro or extra-uterine life, requires to undergo certain changes at every minute period of that life—and those changes it experiences through the influence of the blood of the mother. (Magendie, Mende, Pockels, Baer, Chaussier). 91. The function of the Placenta, therefore, seems to be to facilitate, and in good truth to effect, the necessary changes in question (90). The decidual vessels receive the arterial blood of the mother. This is spread over a very considerable surface of tubular structure, which being, in its distribution, made to come in apposition with the infinite ramifications of the umbilical placental vessels, at innumerable points, (like the inspired air distributed through the bronchial passages is made to come in apposition with the myriads of vascular rami of the lungs); the required changes in the blood of the fœtus are produced, just as the changes called for in the pulmonic blood, are produced by the peculiar arrangement of that part of the animal economy. When the arterial blood of the mother has produced the desired effect on that of the fœtus—it is returned by the decidual veins to the uterine sinuses applied, like absorbing mouths, to the surface of the decidua, when it enters into the general venous system of the mother (Magendie; Personal Observations). 92. Of the two vascular systems, forming the machinery of the utero-decidual and fœto-placental circulation, (82 and 77,) that of the former is the smallest and least extensive (See injections in all the Museums). In point of origin, also, the latter has precedence. The Umbilical vessels unite the fœtus to the chorion before any regular placenta is formed outside of the membrane. Up to the tenth week the decidual vessels are as yet slender, straight, and insulated; while the umbilical vessels begin already to arrange themselves into minute cotyledons[21]. The reason of these arrangements is obvious. The embryo needs growth. This it gets through the maternal blood. But as its gossamer frame, for the space of two and three months, requires but little assistance from such a source for the promotion of growth; and as a large supply of maternal blood, at that early period, would overwhelm, instead of enlarging, the embryonic fabric, such an arrangement of means only obtains, in the reciprocal circulation, as is calculated to ensure the desired effect (Personal Observations).