CARCINOMA OF THE THYROID IN SALMONOID FISHES. 
379 
they are typically spherical, subspherical, or slightly elongated, and consist of a single 
layer of flattened epithelium, with a large lumen completely filled with homogeneous 
colloid readily stainable by cytoplasmic stains. The cells of this epithelium are poly- 
hedral, very flat, with very deeply staining slightly vesicular nuclei, lying parallel with 
the longer dimensions of the cell. The nucleus is longer than wide, about as thick as the 
short diameter of the cell, but its other dimensions are shorter than those of the cell. 
In size, alveoli show wide variation, the smallest measuring as little as 0.02 millimeter, or 
even less, in diameter. From these all sizes are to be found up to plainly macroscopic 
follicles at least 0.75 millimeter across. Perhaps even larger ones occur. Most of the 
alveoli of course fall well within these extremes. Only the largest are visible to the naked 
eye. 
There is thus a promptly recognizable similarity between the structure and appearance 
of the thyroid unit in the trout and that of man and other mammals. The entire gland 
in these groups is similarly comparable in location, function, and perhaps in size, though 
it is impossible to weigh the trout thyroid and difficult to estimate its weight or bulk. 
The chief difference between the gland in fish and mammals lies in the absence from the 
fish thyroid of even a suggestion of a capsule, which in man is a definite and important 
structure which completely delimits the thyroid from the neighboring tissues and 
confines its units as a definite gland organ. The trout thyroid lies in the various tissues 
beneath the floor of the mouth without any very definite interstitial tissue of its own. 
For the normal structure of the thyroid we have examined trout taken from natural 
waters of the country, such as streams and lakes in wild and unsettled regions. From 
these it is apparent that there is no inconsiderable variation in the thyroids of adult fish 
in their natural habitat. This variation certainly depends largely upon age and season, 
is no doubt in part individual, and is in our opinion also a function of other causes among 
which is the nature of the waters inhabited, whether shallow, rapidly flowing streams 
or the more quiet and deeper lakes. These natural conditions react with the metabolism 
of the fish. 
The simplest condition of the thyroid, as described above, was seen in wild brook 
trout taken in January from the Au Sable River in Michigan. (Fig. 17.) The same 
species taken in June from streams in Wisconsin differ in showing somewhat higher 
epithelium, which may be regarded as cuboidal. Our specimens from these two sources 
show the simplest or lowest epithelium, in most cases flattened and never higher than 
cuboidal. Specimens taken in May from a lake in Algonquin National Park, Ontario, 
Canada, show a further advance. The epithelium is distinctly higher and the general 
character of the thyroid picture seems to us to separate these fish from the other groups 
of wild fish discussed above and to justify their consideration under the heading of 
simple hyperplasia. 
SIMPLE HYPERPLASIA AND COLLOID GOITER IN WILD AND 
DOMESTICATED FISH. 
The picture of the normal thyroid in the wild Salmonidae is extremely characteristic, 
and although, as described, considerable variations are found in the size of the alveoli, 
