CARCINOMA OF THE) THYROID IN SADMONOID FISHES. 
449 
Aside from the actually observ’ed subsidence of visible tumors under some abate- 
ment of the conditions of domestication, trout taken at random from fish-cultural ponds, 
with clean thyroid region, show the usual microscopic picture associated with thyroid 
known to have recovered from tumor formation. The wild trout used in feeding experi- 
ments showed numerous examples of this sort. (Fig. 91-92.) That trout can recover 
under apparently the same conditions under which they acquire the disease, and under 
which other trout are showing progressive tumors, seems sufficiently attested by our 
experience. 
Recognition of spontaneous recovery is a necessary factor in studying the controls 
of fish treated with various metals. It is a remarkable fact that in only three or four 
instances in a total of 46 controls did we find spontaneous recovery under the conditions 
selected for these experiments. Regression does not appear to occur until after the dis- 
ease has continued for some time. The best evidence of regression from moderate hyper- 
plasia is found in the wild Wisconsin brook trout with selected feeding, in tanks i and 2 
containing lots 2146 and 2147. (See table viii, p. 100.) 
In this experiment at the end of one year, fish fed upon raw liver and raw ox heart 
showed marked hyperplasia, whereas those fed upon various other types of food showed 
no evidence of hyperplasia. At the end of two years the only lots of fish remaining free 
from evidence of hyperplasia were those fed upon natural food (i. e., live maggots, fresh 
water and salt water mussels, etc.), whereas those which had from the beginning been 
fed upon raw liver or raw ox heart in the various experiments, showed a few individuals 
with well-developed visible tumors and a large number with well-defined evidences of 
retrograding hyperplasia. A microscopic examination of various fish from these lots 
will serve to characterize the changes which indicate regression of the earlier stages of 
the disease. See 2099, 2101, 2103, 2104, on pages loi and 102. 
Summed up, these changes are the evidences of an increased amount of thyroid 
tissue indicated by the presence of follicles in regions in which they do not normally 
occur. The follicles are not closely packed as in active hyperplasia, there is no evidence 
of hyperemia, there is a well-defined stroma, the stroma often forming trabeculae between 
the alveoli as wide or often wider than the diameter of the follicles. The follicles them- 
selves show great variety in size, a few larger ones are filled with stainable colloid, but 
the majority of them are small, of irregular shape and free from colloid or are but partly 
filled with poorly stainable colloid. The epithelium is flattened, often definitely atrophic, 
the protoplasm greatly reduced in amount, so that the cells are composed for the great 
part of their nuclei, which stain homogeneously and deeply. (Fig. 91.) Frequently 
evidences of there having been more than one layer of epithelium may be found. The 
greatest changes are in the peripheral regions, which is characteristic of the regression 
of all neoplasms. The evidence of the previous branching and irregular tubular type 
of proliferation, which characterizes the growing tumor, are found in atrophic struc- 
tures of tubular and branching shape, sometimes reduced to a mere cord of cells. (See 
fig- 92.) 
From a survey of such specimens it is clear that an actual disappearance of thyroid 
tissue must have taken place. In the earlier stages of regression from large tumors 
