American Fisheries Society 379 



tumor, while it may incline to one side or the other, origi- 

 nates on the median line. The body of large tumors may 

 have grown wholly toward the left or right side, but will 

 be found to spring from the middle region. In rare cases 

 no part of a tumor is in contact with the median line but 

 arises from an isolated particle of thyroid out on one of 

 the gill arches. 



The tumors swell out into the mouth and throat, regions 

 well covered and protected and but little visible to a casual 

 observer without the fish in hand. Tumors are sometimes 

 visible when the fish is in the water, but usually this is 

 not the case. To find them the fish must be taken up, the 

 mouth opened and the cavity examined, the gills then widely 

 spread and the throat examined from below. Tumors of 

 considerable size are often present but unsuspected from the 

 appearance of the fish with the mouth nearly closed and 

 the gills folded. It is in fact surprising what a considerable 

 percentage of visible tumors may exist in a brood of trout 

 without the knowledge of the fish culturist in charge 

 of them. 



Not all tumors, however, originate within the thyroid 

 region proper, that is, internally. Besides the few occurring 

 on the gill arches, the tip of the lower jaw is an occasional 

 location. The most common external source is the gular 

 pit, a depression formed of infolding of the skin on the 

 median line of the throat. A tumor in this location may 

 be a direct outgrowth from the thyroid region, but it seems 

 certain that many cases originate entirely from the integu- 

 ment at this point and are quite independent of the main 

 deposit of thyroid. In this case they may be classed with 

 those enlargements occurring on the tip of the lower jaw. 

 Whether or not these grow from isolated thyroid deposits 

 has not been demonstrated. 



Most tumors of some size are rather soft and yielding 

 in consistency, and of a reddish hue from the abundance of 

 blood contained. Frequently they are cystic and become 

 abraded and bleeding from scraping against hard surfaces. 



