CARCINOMA OF THE THYROID IN SAEMONOID FISHES. 
401 
junction and in the gill spaces on both sides, protruding between the first and second 
branchial arches, and penetrating the floor of the mouth in the median line at the junction 
between the second and third branchial arches. 
Microscopically, this extensive growth is found to infiltrate the surrounding struc- 
tures, bone, cartilage, and muscle. It may be classified as alveolo-tubular type. In 
many regions it presents a characteristic solid structure. Only in occasional areas do 
a few follicles contain poorly staining colloid. In the region of the third branchial arch 
is found a cross section of bone, in the lumen of which is a group of thyroid follicles of 
strictly normal appearance. (Fig. 47.) The follicles are of the usual size filled with 
homogeneously and deeply staining colloid, the epithelium is flattened, protoplasm 
small in amount, the nuclei stain deeply and homogeneously. A study of the preceding 
and succeeding serial sections fails to show any opening through the wall of bone. The 
tumor tissue is in immediate contact with the shell of bone for a considerable extent of 
its circumference, the remainder is covered with periosteum and dense connective tissue. 
The significance of this finding is that here we have a large tumor of tubulo-alveolo- 
solid type infiltrating the surrounding structures, while lying within the medullary 
space of the bone structure and included in the tumor tissue is a deposit of normal 
thyroid tissue which by its appearance, and the presence of deeply stainable colloid, 
must be in physiological continuity with the metabolism of the fish. That the deposit 
of normal thyroid within the bone has remained unaltered because it was protected from 
an agent working from without by being inclosed within the bone is probable, but no less 
important is the fact that it shows clearly that portions of the thyroid structure in the 
Salmonidae may undergo malignant change, while other portions of the thyroid tissue 
remain strictly normal in appearance and retain their physiological function until, it 
may be said, as this tumor was very advanced, the last stages of the disease. 
A great deal has been said about the significance of the capsule of the thyroid in 
mammals. Here the evidence of malignancy consists in a breaking through the capsule 
and infiltration of the surrounding structures. As the thyroid structure of the teleost 
has no capsule, such a criterion can not be applied, and some have ascribed the infil- 
trative characteristics of these tumors to the isolated character of the follicles and lack 
of a delimiting capsule. One of the best evidences of malignancy in the mammalian 
thyroid is the development of isolated nodules of malignant character within the struc- 
ture of an enlarged thyroid, and here the evidence of infiltration has not to do with a 
capsule, but the infiltration of surrounding thyroid structure. A determination of 
exactly analogous conditions in our tumors indicates that we have in the proliferation 
of the tumor tissue in the Salmonidae an expression of genuine malignancy. 
Infiltration of hone and cartilage . — The growth of these extensive tumors naturally 
leads to the erosion and destruction of cartilage and bone. The mere presence of 
thyroid tissue within the bone spaces of the branchial arches is in itself of no signifi- 
cance. Deposits of normal thyroid tissue are often encountered in the open spaces of 
both bone and eartilage. The shape of the bony and cartilaginous structure in the 
floor of the mouth in the teleost is frequently such that the thyroid tissue grows in 
through small openings in the base of the arches. In many instances, however, we 
