If we now consider briefly the principal results of our investigation, we can, in comparing the behavior of the thyroid after auto- and homeoplastic transplantation, in general recognize three stages. In the first stage, comprising the first 4 to 5 days after transplantation, there is no noticeable difference between the auto- and homeograft; both corresponding pieces behave in the main alike; large parts of both grafts become necrotic in the center; the necrosis begins shortly after the transplantation and concerns only a part of the periphery; here a narrow zone of thyroid tissue is left; it consists of one to two rows of partly well recognizable follicles. After 48 hours the first fibroblasts, polynuclear leucocytes, and lymphocytes appear in the tissue surrounding the grafts. After 72 hours these elements are present also in the center of the thyroid tissue. At the same date the first mitoses appear in the follicular epithelium; their number is, however, as yet small, but it increases markedly during the following days. After 4 days are found the first newly formed follicles, and from now on their number is increasing. Colloid is present only in small amounts in the old follicles. The central necrotic area still persists, but the masses of detritus are becoming smaller, while the number of fibroblasts and lymphocytes in the necrotic part is increasing; new blood vessels are also visible in the necrotic center. The second stage, the stage of transition, extends over the next seven days. The earliest, at first slight, differences between the auto- and homeograft appear after 5 days; the difference is usually very definite after 12 days. During this time there is an increase in the number of follicles in the autograft and correspondingly a decrease in the size of the necrotic central area, which is caused by a gradually progressing removal of the necrotic material in the center of the piece. The follicles are lined with a high cylindrical epithelium, which contains numerous mitoses. The number of mitoses increases markedly after the 4th day and reaches a maximum between the 7th and 9th days. Newly produced colloid is always present in the autograft after the 8th day; in the newly formed follicles the colloid contains numerous vacuoles and is almost never retracted, in contradistinction to the solid retracted colloid in the old follicles, which probably was present in the follicles at the time of transplantation. At the end of this period the central part is almost entirely freed from the necrotic masses and is filled with loosely arranged connective tissue cells, between which isolated lymphocytes and polynuclear leucocytes can be found. It contains also numerous blood vessels and brown blood pigment cells. After 5 days, with the beginning of the second stage, the first as yet rather insignificant differences become noticeable in the homeotransplants. They are as follows: (1) The number of lymphocytes is much larger in the homeo- than in the autograft. (2) The fibroblasts are increased in the central area and these fibroblasts form here very soon firm connective tissue bundles; they surround also the individual follicles or small groups of follicles and form fibrous bands, which are at this stage still very small. During the following days the number of lymphocytes increases and the fibrous connective tissue becomes more prominent, so that 7, 9, 10, and 11 days after transplantation numerous follicles are not only destroyed by the surrounding and invading lymphocytes, but in addition a part of the follicles is encircled and compressed by wide connective tissue bands. This compression is especially distinct in the central parts of the homeografts. At this stage the absence of a well developed vascular system is already noticeable in the central parts of the homeografts. It is especially noteworthy that the follicles of the homeograft which escaped destruction by the lymphocytes and connective tissue are as well preserved as in the autograft. Colloid is usually present in smaller amounts than in the autografts, even in the well preserved follicles of the homeografts. It must, however, be stated that some variations occur in this stage in the degree of destruction of the homeotransplanted pieces. Side by side with pieces in which a marked destruction has taken place, there are other pieces which suffered as yet relatively little. But some of the above mentioned differences between the auto- and homeotransplants are always present to a smaller or larger extent. There are also certain variations in the relative strength, with which lymphocytes and connective tissue injure the follicles; in some homeotransplants the attacks on the part of the lymphocytes prevail, while in others those on the part of the connective tissue are more prominent. The homeotransplants, which are relatively little injured at this stage are spared only to be subject to a stronger attack on the part of the lymphocytes and connective tissue during the following stage. The third stage begins approximately with the 12th day. From now on the difference between auto- and homeograft is sharply defined. In the autografts the regeneration of the thyroid tissue is steadily progressing and is nearly complete after 21 days. After this date the autograft represents throughout the picture of the normal thyroid gland; well developed follicles filled with colloid are surrounded by a very scant connective tissue. Mitoses, which begin already to decrease after the 9th day, are still present at the end of 17 days, but absent after this date. The small amount of loosely built, usually centrally located connective tissue, has no tendency to undergo a fibrous or hyaline change and does nowhere exert a compression on the follicles. It contains always a large number of blood vessels and only a few lymphocytes. In the homeograft the secondary destruction of the follicles is progressing with great intensity after the 12th day. In...