CARCINOMA OF THE THYROID IN SALMONOID FISHES. 405 



a careful study of the development of the thyroid in the embryo gives no indication 

 whatever of the likelihood of thyroid tissue being deposited at this point, and in the careful 

 survey of the distribution of the thyroid in wild fish we have never seen any deposits of 

 normal thyroid tissue even so far forward as the base of the tongue. We must, there- 

 fore, conclude that there is no evidence that thyroid tissue is ever laid down in this 

 locality. 



The anatomical structure at this point is also very unfavorable for the deposit of 

 normal thyroid tissue. The symphysis of the dentaries here is covered only by the peri- 

 osteum, a thin layer of dense subcutaneous connective tissue upon which rests directly 

 the elastic structure and the epidermis of the outer covering. In fish 61, diagram i, 

 we find an extensive growth of thyroid tumor springing from the apex of the lower jaw, 

 entirely distinct and separate from the usual growth of tumor about the aorta and 

 extending up into the floor of the mouth. A microscopic examination of the growth of 

 the tip of the jaw and the primary growth in the substance of the isthmus shows that 

 they present much the same appearance, being both alveolar in type and many of the 

 alveoli containing stainable colloid. (Fig. 52 and 53.) The similarity in the primary 

 and the detached growth on the tip of the jaw naturally indicate that we have here to 

 deal with a metastasis. 



There is, however, a further possibility, and this is that a growth has developed 

 from an implant finding lodgment upon the prepared site of an injury. We have fre- 

 quently observed that fish with large protruding tumors in the gill region attempt to 

 free themselves of the outgrowths by rubbing the tumor upon the bottom of the pond. 

 This is evidenced by the frequent observation of tumors which have been worn off in this 

 way. It is quite easy to conceive that fragments of such eroded tumors floating in 

 the water of the pond might find lodgment upon the injured apex of the lower jaw. If 

 this was the origin of this outgrowth on the tip of the lower jaw, the similarity between 

 the primary tumor and the secondary tumor would have to be a matter of coincidence. 

 We feel we may safely conclude that the growths on the tip of the lower jaw are true 

 metastases from primary growths in the usual locality, the lodgment of the cells by way 

 of the circulation being favored by reparative processes at this point, or that the growth 

 is a result of implantation through the medium of the water. From the standpoint 

 of determining the true neoplastic character of these tumors, the evidence would be in 

 either case of about equal value. 



A case of still greater importance has to do with a 2-year-old brook trout from the 

 same source, in which a tumor measuring 7 by 8 by 14 millimeters, of pear shape, soft 

 consistency, and dark pinkish color, was found in the lowermost portion of the hind gut 

 or rectum just within the anus. (Fig. 25.) Careful examination shows that the tumor 

 is for the greater part covered by the mucosa of the intestine. It protrudes within the 

 intestinal canal, which is greatly flattened and pushed to one side, and has obviously 

 been obstructed by the tumor. (Fig. 54.) A section made from the wall of the intes- 

 tine, including the intestinal lumen, well into the tumor, presents on microscopic exam- 

 ination the following conditions : 



