a diseased mother will exhibit a like immunity. Here the toxins, soluble substances, passing, even when there is no breach of continuity in the vessel walls, from the diseased to the healthy individual, cause a reaction in the phagocytes of the latter by virtue of which they are enabled, if the microbes at any future time find entrance, to destroy them.
So also as regards other zymotic diseases, small-pox, measles, &c., against which immunity may be acquired; while it is certain that a parent who has acquired immunity does not confer it on the offspring which subsequently arise from his or her germs, it may safely be prophesied, though I am not aware that the phenomenon has actually been observed, that a pregnant mother, suffering from any such disease, if she recovers and does not abort, will confer immunity on her fœtus of as enduring a nature as it would acquire did it take the disease itself after birth and recover. Mr. Hutchinson speaks of immunity acquired in this way as an unexpected fact; I think, however, we may claim that the fact is just such as might be expected. The only thing puzzling about the process is the circumstance that an individual, in whom the toxins alone are present, acquires immunity so much more quickly, and with so much less injury, than an individual in whom the microbes are present as well as the toxins; for instance, a non-infected mother, pregnant of a syphilitic embryo, acquires immunity with far greater speed, and with far less harm to herself, than a woman who is actually infected, and in whom the microbes are present as well as the toxins. The explanation of this is probably found when we remember the great length of the incubation period, and of the general duration of the disease. This seems to imply that the toxins of syphilis are of feeble virulence, a conclusion which is further borne out by the circumstance that the general
