40 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



There follows a second stage, one of transition, extending from the 5th to the 

 12th day, in which the formation of acini is less in homoiogenous than in 

 autogenous tissue; likewise, colloid is newly produced in lesser amounts in 

 homoiogenous than in autogenous grafts and this may perhaps be at least 

 partly due to injury inflicted by the homoiotoxins rather than by the lympho- 

 cytes of the host. During this period, and still more so during the third period, 

 beginning after 12 days, there is an increase in lymphocytes and connective 

 tissue in the homoiotransplants ; the fibroblasts tend to produce fibrous tissue, 

 which separates groups of acini as well as isolated acini and exerts pressure on 

 them ; the lymphocytes accumulate in increasing numbers in the homoiotrans- 

 plants and invade and destroy the acini. In the autotransplants, wider blood 

 capillaries grow through the thyroid ring into the center and here the zone of 

 myxoid connective tissue develops, which we have already mentioned, and it 

 surrounds the central fibrous tissue, which is either converted into loose con- 

 nective tissue by ingrowing capillaries and fibroblasts or is expelled into the 

 surrounding tissue. At the same time, fat tissue may be pushed into the center 

 of the thyroid from the outside and fat cells may be observed, although very 

 rarely, also in the lumen of an acinus. This distribution of thyroid acini in the 

 fat tissue is not found in homoiotransplants. At a later period, as, for instance, 

 five months after transplantation, the autotransplanted thyroid may be almost 

 like the normal, non-transplanted gland. Dense fibrous tissue separating the 

 acini, as well as collections of lymphocytes, is lacking. The center of the graft 

 consists merely of small amounts or strands of loose connective tissue. How- 

 ever, mitoses are not frequently seen in such transplants. The greatest number 

 of mitoses is found from 7 to 9 days following autotransplantation and some 

 mitoses may be found at this time even in homoiotransplants; they may still 

 be frequent in autogenous transplants during the later days of the second 

 week, but are lacking or very rare after 17 days. 



In discussing the variable accidental factors which, apart from the nature 

 of the individuality differentials, may influence the result of transplantation 

 of various organs, we mentioned the fact that if together with the thyroid 

 gland much fat tissue is transplanted, a great part or even the whole of the 

 transplant may become necrotic. This seems to be true, however, only of 

 homoiotransplants. The same factor interferes with autogenous transplants 

 much less seriously ; it appears that in the latter a partial necrosis caused by 

 the fat tissue, may later disappear, as a further demonstration of the great 

 power of adjustment possessed by autogenous tissue. 



We find, in guinea pigs, that younger hosts react on the average in a milder 

 way towards homoiogenous transplants than do older animals ; thus in grafts 

 made in hosts about 10 days old the preservation of the thyroid gland is, on 

 the average, somewhat better and the formation of fibrous tissue in the trans- 

 plant less extensive than in older hosts. Moreover, the lymphocytic reaction 

 may be less severe; but if the preservation of the thyroid tissue is relatively 

 good, the lymphocytic reaction may be quite intense in the young guinea pigs 

 as well. Some differences between younger and older hosts may be noticeable 

 between 11 and 17 days after transplantation. While as a rule also in younger 



