CARCINOMA OF THE THYROID IN SALMONOID FISHES. 
427 
observation had not been fed artificial food but had found a sufficient supply of natural 
food in the grass-lined pond they occupied. Lot 1939 A, 230 two-year-old brook trout, 
and lot 1939, 132 two-year-old brook trout, were hatched from the same lot of eggs, 
divided and placed in their respective ponds during the summer of 1908. They were 
comparable in every respect except their location in the ponds, the feeding for all the 
lots being the same, i. e., raw liver. Lot 1939 A occupied pond 12 next to the uppermost 
pond; 1939, pond 15, three ponds below. The chart shows that 1939 A showed 3 per 
cent of visible tumors, being the lowest incidence of any lot affected, 1939 15 per cent. 
Between these ponds lot 1937 in pond 14, also 2-year-old brook trout, gave an incidence of 
8.7 per cent. Lot 1986, humpback salmon, were yearling fish and in common with the 
hybrid salmon, lot 1994 of the same age, showed the highest incidence of any of the 
lots, namely, 84 and 92 per cent. Lot 1950, two-year-old landlocked salmon in pond 10, 
were the least favorably placed 2-3'ear-old fish and showed the highest incidence for this 
age, 24 per cent. (Fig. 4b.) 
Here we may plainly see that the incidence of the disease increases from above 
downward in the ponds, both in the 2-year-old and the yearling fish. The deduction 
may also be drawn from the conditions found in these ponds that the incidence of the 
disease is greater in the yearling fish than in the 2-year-old, but subsequent observations 
indicate that other factors, such as the species heredity and hybridization, may so 
markedly affect the susceptibility that this material is not suitable for such deduction. 
(See p. 82.) 
Perhaps the most striking feature of this diagram is the evidence it affords of im- 
munity in certain species and certain lots. The 2-year-old Scotch sea trout, lots 1947 
and 1947 A, were found on examination to present no visible tumors and it was con- 
cluded that they were immune to the disease, a fact which microscopic examination and 
further observation has confirmed. The 2-\'ear-old landlocked salmon, lot 1944, occupy- 
ing pond 9, although placed below the yearling hybrid lot, 1994, in pond 6, were also 
free from the disease, although the 2-year-old landlocked salmon, lot 1950, immediately 
below them in pond 10, showed the highest incidence of the disease in 2-year-old fish. 
Yearling hybrid salmon, lot 2017, were also free from the disease and in accordance 
with the general scheme, the smallest incidence in susceptible hybrid salmon, lot 1995 
in pond 4, is found in the uppermost pond. The further histories of the hybrid lots 
and the immune fish are traced in the observations of the succeeding years. 
An attempt was made to trace the history of these fish from the time they were 
hatched in the hatchery through the various troughs which they occupied at different 
periods up to the time of their final location in these ponds. The records pertaining 
to the various lots covered the origin of the eggs, the time of hatching, the mortality 
at ever\" stage of their history in the hatchery, the temperature of the water taken three 
times daily, and exact data as to the feeding, when first begun, and the amounts fed. 
While it was impossible to determine the exact troughs in which, or the exact time when, 
the fish might have contracted the disease, yet our attention was finally fixed upon a 
