CARCINOMA OF THE THYROID IN SALMONOID FISHES. 449 



Aside from the actually observed 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 trabecule 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 



