such a way as to exclude the possibility of the cells of the chorionic villi getting into the blood.
How can we then explain the existence of the defensive ferments during pregnancy? It can only be in very exceptional cases that they are produced by the invasion of morphological elements. In most cases it must be the result of the transfer of particular bodies—the constituents of particular cells, or the products of their decomposition. It may be, of course, that the extraordinarily active metabolic processes, which arise at the junction of the maternal and fœtal organisms, result in an insufficient reduction of many of the products of the cells of the placenta; the metabolism, in short, overreaching itself through its own rapidity. It is, however, also conceivable that the cells themselves break up easily.
The following view is probably the correct one. The organism of the mother has at its disposal, up to the appearance of pregnancy, a certain amount of cells of a certain kind, which all harmonize, in their metabolism, with each other. Now, with conception, comes the appearance of an entirely new kind of tissue, which has to perform particular duties. Although the impregnated egg and the developing placenta, with all its various cells, are in harmony with the species, nevertheless the metabolism of all these cells appears as something quite new and strange to the complex of cells composing the
