404 
BUIvIyETiN OF THE BUREAU OF FISHERIES. 
homotypic character; and that in others the great variation in size of cells and marked 
deviation from the organal type produce tumors of heterotypic character. 
IMPLANTATIONS AND METASTASES. 
It is obviously of the greatest importance in a neoplasm of the character of the 
carcinoma of the thyroid in the Salmonidae to determine whether or not metastases 
or possibly implantation may occur. Although infiltrative growth is and will always 
remain one of the best evidences of malignancy, yet in the last analysis a true neoplasm 
must present some evidence of metastasis formation or the development of implants. 
The experimental study of cancer has placed a new significance upon the importance of 
transplantabihty. Experimental results with mouse and rat tumors clearly indicate 
that metastasis formation may be controlled or at least influenced by concomitant 
immunity, and as the immune phenomena are more outspoken in the more virulent 
types of cancer, it is not surprising that a neoplasm involving a vital organ like the 
thyroid, through its early infiltrative growth involving vital structures, might prove 
the determining factor in the early carrying off of the individual and thus the life of the 
affected fish might not extend into the period in which metastasis formation more 
frequently occurs. 
The rarity of the occurrence of me’tastases in any given group of tumors may well be 
expected. In such a case one or two instances may serve the purpose of definitely 
fixing the nature of the neoplasm. The occurrence of metastasis in the thyroid carci- 
nomata of fish is certainly rare. The circulation of the fish is not well adapted to the 
transportation of cells. The region in which transported cells would most easily obtain 
lodgment would be in the bifurcations of the vessels of the branchial arches, and it is a 
common occurrence to find isolated growths well away from the median line on these 
structures, but as deposits of thyroid tissue are located immediately about the aorta 
many of these growths will be found to be simply outgrowths from the primary mass in 
the median fine. Some of them are, however, so widely displaced from the median mass 
that they may be looked upon as regional metastases in which the transport of cells 
would have to be accomplished through the lymph channels. It may be said that 
growths in this region, however, are not competent to determine the question of true 
metastasis formation from transport of cells. A region already referred to and one in 
which the development of tumors is very frequent, occurring in not less than 25 per 
cent of the studied cases, is the jugular pit. The explanation for these growths is, 
however, found in the frequent presence of misplaced thyroid tissue. The origin of 
these deposits has been clearly traced and adequately explained. 
Early in our observations we noted that occasional growths upon the tip of the lower 
jaw were to be seen. (Fig. 28.) This region in the fish is one peculiarly exposed to 
injury. The fish confined in tanks almost certainly run into the sides of the tanks or 
the screens and injure the epidermis at this point. These growths at the apex of the 
lower jaw might be explained by the presence of unusual deposits of thyroid tissue at 
this point. We have examined, by serial section, this region of the lower jaw in 25 
fish, and have never found any trace of normal thyroid in this location. Furthermore, 
