450 
buIwLKTin OB The; bureau of fisheries. 
evidences of extensive hemorrhage are found, a characteristic which also distinguishes 
regression following treatment with metals. In such cases organization of the areas of 
hemorrhage by connective tissue appears. (Fig. loi.) In the earlier stages of regression 
the periphery of the tumor will frequently show evidences of round-celled infiltration 
in the stroma. The scirrhous appearance of advanced tumors which have undergone re- 
gression is distinctly characteristic. It will be seen that these changes are distinct from 
the changes following so-called resolution of hyperplastic thyroid which leads to the 
stage of colloid goiter, as observed in the Scotch sea trout, where the terminal stage of 
simple hyperplasia is the large follicles filled with stainable colloid, lined by flattened 
epithelium. (Fig. 22.) The changes here described as regressive are of the same nature 
as those encountered in the regression of transplanted or spontaneous carcinoma of the 
breast in mice. (Gaylord and Clowes, 1906.) The microscopic picture of retrograding 
large tumors in no way differs from the changes following regression by treatment with 
iodine, arsenic, or mercury, except that no doubt in the latter instance the changes pro- 
ceed more rapidly and with less evidence of stroma formation. (Fig. 109.) 
IMMUNITY. 
Among marine species it has been remarked that the chinook salmon held at the 
Craig Brook station until 3 years of age were entirely immune to visible thyroid growth. 
These fish were nevertheless in poor condition, and did not grow and thrive normally. 
This fish is anadromous, and under natural conditions the young, being hatched in fresh 
water, soon find their way to sea. Notwithstanding this, they may survive and grow 
to maturity when held captive in fresh water. It might be inferred that the immunity 
which resides in marine species of salmonoids, or the protection which the sea affords 
them — no marine species having been found in the sea with thyroid enlargement — is 
transmitted through the eggs and continued during a prolonged sojourn in fresh water. 
But the humpback salmon is likewise a Pacific marine species, and yearlings of this kind 
produced at Craig Brook are fine examples of thyroid tumors. Of 319 fish (lot 1986) 
about I year old, 14 per cent bore tumors when examined in April. By the following 
August only 79 fish remained, of which 84.8 per cent had tumors. These resembled the 
tumors of the Pacific hybrid salmon and showed the same structural evidence of malig- 
nancy. Wilkie reported (Gilruth, 1902) a thyroid tumor in a 5-year-old Atlantic sal- 
mon in fresh water in the ponds of the Clinton hatchery. New Zealand, and we have 
seen one example at the Craig Brook hatchery in Maine. It is evident that marine 
species may develop thyroid carcinoma while resident in fresh water. 
The best example of species immunity is afforded by the Scotch sea trout, which at 
Craig Brook is almost completely immune either to visible or microscopic enlargement. 
One doubtful case of a distinct tumor is recorded and several of red floors, at 3,4, and 5 
years of age. Doubtless trout of this species can not continue indefinitely without 
affection under the conditions which produce the disease. 
While the Pacific salmon hybrids all show a high susceptibility, other hybrids and 
the brook trout species exhibit various degrees of susceptibility and immunity to tumor 
formation. The landlocked salmon is about as susceptible as the brook trout, but lot 
