continuance leads to the appearance of insufficiently decomposed products. Involution, too, may be the cause of the formation of products that are out of harmony with the blood; in which connection we have in mind more especially the degeneration of the thymus, and the climacterium.
Many of our own researches, and those of various observers, have shown that, during the whole period of pregnancy, defensive ferments circulate in the blood, which are able to reduce placenta albumen. These ferments may be demonstrated within about eight days after impregnation. Their presence is, without any doubt, dependent upon the circulation of disharmonious substances originating in the placenta; since the defensive ferments disappear within fourteen to twenty-one days, when the relations of the placenta with the maternal organism have ceased.
Attempts have also been made to decompose placental tissues by means of the blood serum of the fœtus. No digestion could be induced; nor can the serum of pregnant individuals be made to attack the tissues of the fœtus. In any case these observations must be carefully followed up. It might be imagined, a priori, that there are developmental stages, in which the tissues of the fœtus are as yet so little differentiated, that they are still of a generalized character. Nor did umbilical blood
