38 o 
BULLETIN OF THE BUREAU OF FISHERIES. 
there is but a very limited range of change in the epithelial cells. So characteristic is 
this picture that it is possible to recognize the condition of simple hyperplasia of the 
thyroid, although the exact relation of simple hyperplasia to the function of the organ 
is not clear. We have encountered a condition of the thyroid in specimens of brook 
trout taken from the Algonquin National Park in Ontario, Canada, where the change 
is sufficiently marked to constitute, in our opinion, a condition of simple hyperplasia in 
fish living under mid conditions. (Fig. i8.) The specimens were taken in the regular 
course of angling on several days in Little Island Lake in the park. The picture pre- 
sented by the thyroid of these fish is sufficiently characteristic to permit the description 
of the thyroid of any one specimen to serve for the rest. As compared with the thyroid 
structure of Wisconsin and Michigan wild brook trout, the alveoli lie more closely packed 
and present much more the appearance of a definite organ than in the strictly normal 
condition. The alveoli are of much more irregular shape, presenting irregular forms 
and protrusions, smaller pouch-like additions to the typical spherical or oval alveolar 
structure, and evidences of budding. The epithelium is high cuboidal, stains more 
deeply, and the long axis of the deeply stained nucleus is usually perpendicular to the 
circumference of the alveolus. The colloid is diminished in amount, many of the 
smaller alveoli containing little or none. It stains less deeply than in the normal struc- 
ture. There are evidences of hyperaemia in the stroma and the stroma is much richer 
in small cells than in the normal. The total number of alveoli seems to be increased. 
We have not seen any evidences of karyokinetic figures in the cells. The epithelium 
never reaches the high columnar and bizarre shapes found in the early stages of car- 
cinoma. The entire thyroid structure is more uniform in type. 
Simple hyperplasia of the thyroid has been described by Marine and Lenhart (1910) 
as occurring in fish obtained from Lake Erie. Pike and bass, according to these authors, 
are commonly affected; sheepshead and herring more rarely. They also report an 
example of spontaneous colloid goiter in the pike which they consider the terminal stage 
of hyperplasia that has undergone resolution. 
A simple hyperplasia of this type undoubtedly occurs in fish under domestication. 
It is impossible to distinguish simple hyperplasia in fish hatcheries from the first stages 
of carcinoma of the thyroid, a difficulty which is common to the diagnosis of all malignant 
tumors. We have been fortunate in having for study at Craig Brook a variety of the 
Salmonidae which possesses natural immunity to carcinoma of the thyroid. These fish 
are the Scotch sea trout {Salmo trutta Linnaeus). The original stock was obtained from 
abroad as eggs, hatched in the hatchery, carried through its various troughs, and the 
adult fish ultimately came to occupy the ponds lying in a position where they received 
the water from ponds above, in which carcinoma of the thyroid in brook trout was 
extremely prevalent. The facts bearing on the evidences of immunity in this connection 
will be considered later. The eggs taken from these domesticated sea trout have been 
again hatched in the hatchery, maintained in the outside smaller troughs, and we have 
thus had an opportunity to study the offspring of the adult fish at various stages, as 
well as the condition of the thyroid in the larger fish. We have never found macroscopic 
evidence of even the earliest stages of carcinoma of the thyroid in the younger of these 
