380 Fortieth Annual Meeting 



In size they sometimes become relatively enormous, approxi- 

 mating the size of the head of the fish. They may greatly 

 distend the gills and almost fill the mouth, interfering with 

 breathing and eating. These cases are extreme and rela- 

 tively rare. All gradations in size and form down to the 

 barely visible external enlargement and the red floor are 

 seen. 



Microscopically the thyroid tumor has the appearance of 

 a modified thyroid gland. It is not as if the gland had 

 merely grown immensely larger and nothing more. The 

 units which compose the normal thyroid are enlarged and 

 variously changed in shape and in the size and shape of their 

 individual component cells. The colloid substance which 

 fills the spaces of the normal thyroid follicles has decreased 

 or disappeared. The microscopical picture is varied, de- 

 pending on the age of the growth of the portion of the 

 tumor under examination and on unknown factors. This 

 subject is purely technical and needs but brief discussion 

 here. It may be said that the thyroid tends, in its abnormal, 

 lawless growth not merely to press outward and push away 

 surrounding parts, like a ball growing larger, but to pene- 

 trate and invade and to some extent destroy any tissue 

 standing in its way, whether muscle, cartilage or bone. 

 This invasion or infiltration of surrounding tissues is a 

 characteristic mark of cancer. 



The thyroid tumor occurs especially among the salmon 

 family in fresh water. It is best known in the brook trout, 

 or char, and is also common among the rainbow and brown 

 trout, various hybrids of the char, land-locked salmon and 

 in hybrids of the Pacific salmon but not in any sea-run 

 pure salmon species. Scotch Sea trout are practically 

 immune. So far no case in marine fish has been reported. 

 It will have impressed the fish culturists who have noticed 

 the distribution of these tumors that they occur in the 

 salmon family when its members are domesticated in fresh 

 water. These facts appear significant. Whatever they 

 mean they lose something of their apparent importance 



