CARCINOMA OF ThF THYROID IN SALMONOID FISHES. 
451 
1944 showed no visible evidence of thyroid disease until the fourth year, though living 
under the conditions which produced the disease. Lot 2017, hybrids of the brook trout 
and landlocked salmon, when it consisted of 1,553 yearling fish, showed not a single fish 
with a visible process. In the second and fourth years a few visible growths appeared. 
A lot (2133) of rainbow trout, which develops frequent and large tumors at some hatch- 
eries, were held two years at the Craig Brook station without acquiring any visible 
tumors and only a small percentage of red floors. 
As previously referred to, rainbow trout at the Caledonia hatchery in New York 
appeared to have a very low incidence of the disease, about one-half of i per cent of the 
fish each year showing well-developed tumors, this covering a period of approximately 
25 years. As stated, no fresh blood during that period was added to this lot of fish. 
Tumor fish found each year were destroyed and a probable original resistance of the lot 
was protected and perhaps added to by this form of selection, so that, as we have pointed 
out, in the epidemic at the Bath fish hatchery covering a period of two years these fish 
remained practically immune, only i or 2 fish in a lot of 75 adult fish being found with 
tumors in the course of the two years’ epidemic. Exactly the same state of affairs 
existed in a lot of 20,000 German brown trout which had also been held without the 
addition of fresh blood at the Caledonia hatchery. These were represented among 
others by some 200 of their offspring, which as 4 and 5 year old fish went through the 
epidemic without a single tumor. This was not the case with some 4,000 young German 
brown trout which were sent from the Caledonia hatchery to the Bath hatchery as 
young fish, and which toward the end of the epidemic, as 2 -year-old fish, developed a 
considerable incidence of the disease. 
That the rainbow and brown trout retained so .nany years at the Caledonia hatchery 
really possessed a definite immunity against the disease is shown by the fact that during 
this time attempts to rear brook trout to adult age and maintain them resulted in a high 
incidence of thyroid tumors in this species. That a given lot of fish from one hatch may 
possess at a certain age an almost complete resistance to the disease, while another lot 
from another hatch of the same species and of the same age, kept under the same condi- 
tions, may show a high incidence of the disease, is possibly explainable on the basis of 
blood relationship. The manner in which spawn and eggs are taken and fertilized 
would easily bring about the presence in any given lot of a large number of fish with the 
same parents. With the exception possibly of the hybrids, some of the lots of which 
were small, no lot of fish which we have studied could possibly be entirely of the same 
parentage; but as a large number of individuals in each lot are certainly of the same 
parentage it is explainable that the high degree of immunity or susceptibility in a given 
lot may be due to this fact. Such a supposition is in accord with the now well-known 
facts of family predisposition in both goiter and cancer in human beings. 
The fish offer a remarkable opportunity for the careful study of this phase of 
immunity. It will be easily possible to obtain in any hatchery in which the disease 
is endemic, on the one hand from parents both having visible tumors, or, on the other, 
from parents showing distinct immunity, a sufficient number of eggs to hatcli several 
hundred fish. A number of fish could be reared from such lots sufficient to demon- 
