CARCINOMA OF THK THYROID IN SAFMONOID FISHFS. 
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 : 
