4o8 
bulletin of the bureau of fisheries. 
changes leading to carcinoma of the thyroid in the Salmonidae as we have described it. 
In figure 84 the change from flattened to columnar type, with deeply staining protoplasm, 
lengthening and flattening of the tubules, closely resemble those found in the mam- 
malian thyroid by Hitzig and Michaud, and, with the exception of the hyperaemia, 
which is associated with the more intensive chnges, in the hyperplasia of the thyroid in 
the Salmonidae. The advent of isolated nodular growths, sometimes sharply cireum- 
scribed (fig. 45), indicates that focal proliferation of the thyroid tissue in the fish fre- 
quently leads to the development of nodules presenting the picture of nodular struma 
in mammals. The structure of the normal thyroid in the Salmonidae is so simple and its 
amount so limited, that a careful study of this structure in all age periods of the fish 
renders it clear that the advent of tubular structures with columnar epithelium clearly 
represents a pathological change, and here we are not troubled with the many questions 
which arise to complicate the study of these structures in the mammalian thyroid. We 
can, in the thyroid of the Salmonidae, definitely exclude the idea voiced by Kramer, 1910, 
that such tubules in the mammalian thyroid were probably originally the remnants 
of execretory ducts persisting from an earlier period of development of the mammalian 
thyroid. It is plainly evident from the study of the normal thyroid in the Salmonidae 
and the genesis of hyperplasia, nodular growths and fully developed carcinoma, that the 
changes in this organ are brought about by the action of some agent working focally 
upon the epithelium of normal vesicles, and we can clearly exclude all possibility of 
embryonic rests playing a part in the genesis of circumscribed adenomata or cancer. 
The evidence adduced on this subject therefore confirms, so far as the evidence is 
applicable, the conclusions of Virchow, Hitzig, and Michaud that struma nodosa develops 
as the result of focal change in the epithelium of normal structures of the thyroid. The 
production of tubules and irregularly distorted spaces lined with columnar epithelium 
and the process of development of new follicles by budding, as described and illustrated 
by Michaud, are repeatedly encountered in our specimens, especially in the earliest stages. 
(Fig. 36.) The theory that carcinoma of the thyroid develops especially from the 
adenomata of nodular struma and that endemic goiter is the result of a physiological 
hyperplasia of normal thyroid tissue, finds no support in our study of carcinoma of the 
thyroid in the Salmonidae. The theory of Marine that iodine affects alone physiological 
hyperplastic changes of the thyroid tissue and does not affect these adenomata, and may 
thus be used as a means of distinguishing between physiological hyperplasia and cancer, 
is obviously untenable, as we find that iodine, as well as mercury and arsenic, affect 
not only fully developed carcinoma of the thyroid but where tumors contain individual 
adenomata these are likewise affected. Well-developed tumors in the Salmonidae some- 
times closely simulate the structures of nodular struma in the mammal. Figure 65 
represents such a tumor and may be compared with figure 66, struma nodosa in man. 
The tumors of the fish frequently contain the so-called Wachstum centra of Aschoff. 
(Fig. 66.) 
Although it may not be wise to go too far in the comparison of carcinoma in the 
Salmonidae with carcinoma of the thyroid in mammals, yet inasmuch as we will show 
