1. After transplantation of the thyroid of the guinea pig into rats there is a primary injury of the transplant, noticeable as early as 3, 4, and 5 days after transplantation. The tissue is less resistant, and it is preserved only under the best conditions. The number of mitoses is much diminished in the transplant, but they may appear even as late as 9 days after transplantation. This was also the latest time at which living tissue was found. Epithelium is best preserved in the neighborhood of growing fibroblastic tissue, and growing epithelium attracts fibroblasts. Few fibroblasts grow between acini, and they have a tendency to form fibrous tissue. Dense fibrous tissue compresses acini and contributes to their destruction. The vascularization of the graft is very poor, but some capillaries may penetrate between acini. The collection of lymphocytes around acini is only casual and may be found where fibroblasts are active and especially around the blood vessels in the capsule of the graft. On the whole, heterotransplanted tissue does not attract lymphocytes to any marked extent. Connective tissue and lymphocytes contribute only secondarily and to a minor extent to the destruction of the heterotransplant.
2. After transplantation of the thyroid of the rabbit into rats the last mitoses are found 9 days after transplantation; living acini are observed as late as 11 days after transplantation, and ducts of squamous epithelium even somewhat later. Different kinds of tissue seem to show a different degree of resistance to the action of heterotoxins. The difference in the resistance to heterotoxins corresponds to the difference in the resistance of various tissues to other kinds of injurious influences. The thyroid of rabbits is, on the whole, better preserved in the rat than the thyroid of the guinea pig; there is also more mitotic activity in the rabbit thyroid.
The reaction of the tissues of the host towards heterotransplanted rabbit thyroid in principle is similar to the reaction to heterotransplanted guinea pig thyroid.
3. After transplantation of rabbit thyroid into guinea pigs well preserved thyroid is found not later than 8 days, while the last mitoses appear 6 days after transplantation. The number of mitoses is very small. The host tissues again behave in a manner characteristic of heterotransplantation. It is probable that slight infections and the presence of polynuclear leucocytes are responsible for the somewhat inferior results in this kind of heterotransplantation.
4. After transplantation of cat thyroidinto the rat, signs of degeneration in the transplanted acini appear at the end of the 1st week; they increase during the 2nd week. The last well preserved acini are found at 14 days; a few degenerating acini are still visible as late as 18 days after transplantation. Almost all pieces are entirely necrotic in the 3rd week. Mitoses are only found 5, 9, 10, and 11 days after transplantation, and they are present in a limited number. There is a decided lack of good vascularization in the transplants; it is especially noticeable in the 2nd week after transplantation. Fibroblasts penetrate at various places between the acini and occasionally together with a few lymphocytes may destroy some of them. From the 9th day on the presence of fibrous tissue around the acini is noticeable. It compresses the acini and thus contributes to their destruction. Even the heterotransplanted thyroid exhibits a restraining influence on the connective tissue which is greater than that presented by dead material. The behavior of the lymphocytes towards heterotransplanted thyroid of the cat is similar to that towards other heterotransplanted thyroid.