CARCINOMA OP THE THYROID IN SAL,MONOID FISHES. 503 



epithelioma affecting the branchial arches and showed that it was widely distributed 

 among the hatcheries of New Zealand. 



Gaylord began the study of the disease in 1908 and reported evidence pointing to 

 an infectious factor in its causation. Marine and I/enhart, as the result of studies in 

 1909 and subsequently, hold that the disease is endemic goiter and have failed to find 

 any specimens which they recognize as cancer. 



The disease is widely distributed throughout the United States and probably occurs 

 more or less everywhere that artificial propagation of salmonoids is carried beyond the 

 early stages. 



II. The normal thyroid follicles in salmonoids resemble those of the mammalian 

 thyroid, but the gland is not encapsulated and not so definitely confined within given 

 limits. In wild brook trout the largest masses of thyroid follicles are faintly macro- 

 scopic, and all the thyroid tissue is located in the neighborhood of and chiefly dorsal to 

 the ventral aorta between the first and third gill arches, and does not extend laterally 

 along the arches. The distribution is somewhat more restricted than that indicated by 

 Gudernatsch. Anomalous deposits frequently occur beneath the epithelium of the 

 jugular pit, but are rare elsewhere. The thyroid follicles of wild trout are regular in 

 shape, usually spherical or slightly elongate and in the typical or simplest condition its 

 epithelium is flattened or never higher than cuboidal. 



III. Simple hyperplasia of the thyroid is met with in trout living under wild condi- 

 tions. The follicles are increased in number, are more irregular in shape, the colloid is 

 diminished, and the epithelium is in large part columnar. Such a hyperplasia exists 

 also in domesticated trout and is not to be distinguished from the earliest stages of 

 carcinoma of the thyroid. The immune Scotch sea trout as yearlings occasionally 

 exhibit this simple hyperplasia, and a few adults are found with colloid goiter. Sponta- 

 neous recovery from thyroid carcinoma in fish does not result in this picture of colloid 

 goiter. 



IV. The first macroscopic evidence of the disease is usually found in an area of 

 hyperemia on the floor of the mouth (red floor). The first evidence of visible tumors 

 may be found at the branchial junction. Tumors may protrude in various directions, 

 at the branchial junction, in the floor of the mouth, or to either side of the gill region. 

 Independent tumors develop in the jugular pit, a region which frequently contains 

 deposits of normal thyroid tissue. The first microscopic evidence of the disease is 

 found to occur in individual follicles, usually those nearest a large vessel. A small 

 group of altered follicles surrounded by normal follicles is frequently found in the early 

 stages. The epithelium is high cubical or columnar, the protoplasm and nuclei stain 

 deeply. Colloid is diminished or absent, the vessels of the stroma hyperemic. Budding 

 of the wall of the follicle next occurs, forming isolated npw follicles of irregular type, 

 and papillary projections into the follicles. As the gland is not encapsulated, newly 

 formed tissue grows between the muscle planes and fills in the areolar spaces. At this 

 stage karyokinetic figures are common, the epithelium is high columnar, and frequently 

 there are several layers of epithelium in a single follicle. Proliferation may now have 



