382 Fortieth Annual Meeting 



plant the tumor from fish to fish. Both the fluid extract 

 and the undiluted particles of tumor are alike fatal. Thus 

 every fish bearing even a small visible tumor is carrying on 

 its own body many times a fatal dose of poison, which is 

 harmless only because it is withheld from the circulation. 



The beginning of enlargement occurs in the hatchery dur- 

 ing the first few months of the existence of the fish and 

 practically all individuals are affected. The external ap- 

 pearance is perfectly normal. The visible tumor stage is 

 usually not reached until the fish become yearlings. One 

 brook trout of five months, however, was found dead with 

 an advanced type of thyroid disease. Among the yearlings 

 the tumor is seen upon handling and examination, and they 

 may have tumors of relatively large size. In the older fish 

 the enlargement reaches its greatest development. The 

 percentage of fish bearing visible tumors varies greatly. 

 Four per cent has been observed among brook trout yearl- 

 ings, and 30 per cent among older ones and these are 

 probably not extraordinary cases for this species. In acute 

 outbreaks nearly every fish may show tumors. Hybrids of 

 certain of the Pacific salmon, as the blueback and hump- 

 back, in process of rearing seem to be especially susceptible 

 and the disease runs through them like an epidemic. In 

 one case 16 per cent of visible tumors in April increased to 

 92 per c^nt by the following August, with an accompanying 

 heavy mortality. Humpback salmon yearlings in fresh 

 water at a hatchery showed the same involvement and rapid 

 course. If we add to those showing visible tumors the 

 number showing the red floor only we get a substantial 

 increase, the total indicating the number showing some 

 visible evidence of thyroid enlargement. Brook trout 

 adults frequently have half the fish or more in this category. 



The effect of the tumor is obviously in part mechanical. 

 It is difficult to separate the effect of mechanical inter- 

 ference with respiration and feeding from the systemic or 

 constitutional effect of an internal secretion capable of 

 bringing about the same anemia and inanition that follows 



