450 BULLETIN Of THE BUREAU OP 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. 101.) 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 



