To us these observations and views had this much value: that they directed our attention to the fact that, during pregnancy, the appearance of substances, that were in harmony with the species but not with the plasma, might be possible. If our view is correct, that the animal organism sets free ferments of a special kind, as soon as material that is in harmony with the species, but not with the plasma, passes into the blood, then it ought to be possible to demonstrate the existence of such ferments during pregnancy. As, however, the ferments, as we know from experience, disappear fourteen to twenty-one days after the actual introduction of disharmonious substances, it was scarcely possible to expect that defensive ferments should be met with during the whole period of pregnancy. One had to experiment with a great many cases, before striking that one in which dissolution of the cells of the villi had just taken place.
Experience, however, soon showed that the serum of pregnant individuals always contains defensive ferments which are directed against placenta albumen. We cannot, therefore, maintain that the tearing off of the epithelium of the chorionic villi is the only cause of the appearance of the defensive ferments. It must be remembered that even the mare exhibits, during pregnancy, defensive ferments which are directed against placenta albumen; yet the placenta of the mare is related to the circulation in
