148 LEO LOEB. 



ment of the peripheral thyroid tissue and of the good vascular 

 supply the normal organ structure is more and more reestab- 

 lished. This occurs within three to five weeks after transplanta- 

 tion. Lymphocytes are at no time prominent in the transplant. 



After homoiotransplantation of the thyroid gland the first 

 stages are similar to those observed after autotransplantation ; 

 but soon two important differences become noticeable. The well 

 developed ring of lymph and blood vessels in the peripheral part 

 of this center is much less prominent after homoiotransplanta- 

 tion ; the connective tissue ingrowth, on the other hand, is much 

 more marked. The whole center becomes soon converted into a 

 dense fibrous mass, much larger in volume than in the autotrans- 

 plant. It may still be well supplied with vessels, but they are less 

 prominent than after autotransplantation. In contradistinction 

 to what happens in the autotransplant this fibrous mass is not 

 absorbed and not substituted first by myxoid and later by thyroid 

 tissue. This transplant never attains similarity in structure with 

 the normal gland. Even around the individual acini the fibro- 

 blasts may become active and form fibrillar and fibrous tissue. 

 The activity of the connective tissue may go still further, and oc- 

 casionally fibroblasts may penetrate into the interior of some 

 thyroid acini and help to destroy them. 



The direct destruction of the graft by the host is a very character- 

 istic feature after homoiotransplantation. It is however, not so 

 much the activity of the fibroblasts as of the Imyphocytes which 

 brings about this result. While in the autotransplant the lympho- 

 cytes are practically absent, in the homoiotransplant they begin 

 to appear as early as five and seven days after transplantation. 

 However, they become more prominent only about nine or ten 

 days after transplantation, and from then on they rapidly in- 

 crease in number in many cases. They approach the transplant 

 mainly by way of the lymph vessels, and in consequence in the 

 homoiotransplant the lymph vessels may become as prominent as 

 if they had been injected. This I found especially noticeable in 

 the rat, on account of the peculiar distribution of the lymph ves- 

 sels at the inner aspect of the thyroid ring. The lymphocytes 

 appear at this place in especially large masses and from here 

 they penetrate in a peripheral direction into the thyroid acini 



