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Show 210 MR. F. M. BALFOUR ON THE [Feb. 1, February 1, 1881. Professor Flower, LL.D., F.R.S., President, in the Chair. The following papers were read :- 1. On the Evolution of the Placenta, and on the possibility of employing the characters of the Placenta in the Classification of the Mammalia. By F. M . B A L F O U R, M.A., F.R.S., Fellow of Trinity College, Cambridge. [Received January 22, 1881.] From Owen's observations on the Marsupials it is clear that the yolk-sac in this group plays an important (if not the most important) part, in absorbing the maternal nutriment destined for the foetus. The fact that in Marsupials both the yolk-sac and the allantois are concerned in rendering the chorion vascular, makes it a priori probable that this was also the case in the primitive types of the Placentalia; and this deduction is supported by the fact that in the Rodentia, Insectivora, and Cheiroptera this peculiarity of the foetal membranes is actually found. In the primitive Placentalia it is also probable that from the discoidal allantoic region of the chorion simple foetal villi, like those of the Pig, projected into uterine crypts ; but it is not certain how far the umbilical region of the chorion, which was no doubt vascular, may also have been villous. From such a primitive type of foetal membranes divergences in various directions have given rise to the types of foetal membranes found at the present day. In a general way it may be laid down that variations in any direction which tended to increase the absorbing capacities of the chorion would be advantageous. There are two obvious ways in which this might be done, viz. (1) by increasing the complexity of the foetal villi and maternal crypts over a limited area, (2) by increasing the area of the part of the chorion covered by the placental villi. Various combinations of the two processes would also, of course, be advantageous. The most fundamental change which has taken place in all the existing Placentalia is the exclusion of the umbilical vesicle from any important function in the nutrition of the foetus. The arrangement of the fcetal parts in the Rodentia, Insectivora, and Cheiroptera may be directly derived from the primitive form by supposing the villi of the discoidal placental area to have become more complex, so as to form a deciduate discoidal placenta, while the yolk-sac still plays a part, though physiologically an unimportant part, in rendering the chorion vascular. In the Carnivora, again, we have to start from the discoidal placenta, as evinced by the fact that in the growth of the placenta the |