CARCINOMA OF THE THYROID IN SALMONOID FISHES. 



397 



From their position they are subject to mechanical erosion on their ventral surface. 

 When well developed the tumor usually fills and completely obliterates the pit. When 

 the latter is not the seat of tumor growth, it is sometimes completely everted by the 

 pressure of the tumor from the main thyroid region, and almost all trace of it lost, the 

 surface of the skin being stretched smooth in this region. 



Table I. — Classification of Visible Tumors by Location. 



visible tumors examined gi 



HISTOLOGY. 



EARLY STAGE. 



Under "Simple Hyperplasia" we have presented evidence which we believe indicates 

 that there occurs in wild and domesticated fish a type of simple hyperplasia which leads 

 to colloid goiter. The first changes in the epithelium in any form of hyperplasia, 

 whether simple or malignant, would be of the same character and thus indistinguishable 

 one from the other. The progress of carcinoma of the thyroid in the Salmonidae may 

 for convenience be divided into three periods: That in which only microscopic evidence 

 of hyperplasia is discernible; the stage in which the growth of tissue extends sufficiently 

 to produce hyperaemic changes visible in the floor of the mouth — i. e., red floor; and then 

 the period of visible tumors. Histologically no fine of demarcation is possible between 

 these various stages. Neither is it possible to distinguish the very first changes in the 

 epithelium at the onset of this disease from simple hyperplasia leading to colloid goiter 

 as we have observed it in the Scotch sea trout which have proven immune to carcinoma 

 of the thyroid. 



Normal thyroid tissue in the Salmonidae is composed of isolated follicles lined with 

 flattened epithelium containing colloid. The follicles are distributed as shown about 

 the aorta in the loose connective tissue. 



The first indication of the disease is found in the hypertrophy of individual cells; 

 in a given follicle usually the change affects one or two adjacent follicles, or the only 

 evidence of the beginning of the disease is found in a small group of follicles Hned with 

 cubical or columnar epithelium in which the colloid is greatly reduced or entirely absent. 

 Hyperemia of the vessels of the stroma is usually present. In our experiments with 

 wild Wisconsin brook trout, in which fish of varying size and age were taken from the 

 wilderness and placed in the waters of the Craig Brook hatchery, the first evidence of 



