376 
BULLETIN OF THE BUREAU OF FISHERIES. 
are here in close relation to and not infrequently lie in actual contact with (fig. lo) the 
cells of this invagination, and thus illustrate the probable origin of pit tumors. One 
or more of the thyroid follicles can be easily dragged away from their neighbors with 
the growth of the pit region and come to lie immediately beneath the pit epithelium, 
as has been remarked in the discussion of distribution (fig. 14). When the thyroid 
later proliferates abnormally, an independent tumor begins to occupy the jugular pit. 
In the adult scarcely any thyroid is located so far forward as in these recently 
hatched fry, where in the neighborhood of the pit it is to be found cephalad of the first 
arch and of the bifurcation of the aorta. The other deposit or aggregation of follicles 
centers in the region occupied by the adult thyroid, or in the vicinity of the second arch. 
They are not arranged with any regularity, were not observed in any case to extend 
laterally upon the gill arches but scatter along directly beneath the mesal bridge, and 
reach in a few cases as far back as the beginning of the fourth arch. Between the 
follicles about the jugular pit and those in the vicinity of the second arch, a gap usually 
intervenes in which no thyroid occurs. The limits of variation in amount and position 
of thyroid tissue can not be defined here, but from the material examined it appears 
that either of the chief groups of follicles as just described may in some specimens be 
entirely wanting, and that the adult gland may develop from one of these groups alone. 
GROSS ANATOMY AND DISTRIBUTION IN THE ADULT BROOK TROUT. 
While the thyroid is not a definite gland unit and on account of its small size, sepa- 
ration into discrete aggregates, and lack of encapsulation can not be extirpated, or 
dissected free in its entirety, it is nevertheless macroscopic in adult trout. On medi- 
section of the floor of the mouth, delicate small masses of tissue may be seen about the 
ventral aorta near the first and second arches between the vessel and the cartilages 
and bone of the mesal bridge into which the gill arches unite. These masses contain 
the chief aggregates of thyroid follicles, and consist in large part of connective tissue. 
They are likely to be found close to the second gill arch, and more likely to lie opposite 
the interspaces between the arches than opposite the arch itself. Under a hand or 
dissecting lens the individual follicles may be recognized embedded in the mass. Only 
the massed follicles are to be seen with the naked eye and therefore the outlying small 
deposits are not demonstrable save on microscopic section. The macroscopic thyroid 
masses in wild fish will only be found close to the middle line and at or just in front 
of or behind the second arch. Such masses closely resemble in appearance the fatty 
and areolar tissues which contain no thyroid, and can not be recognized with certainty 
as thyroid without a lens. 
The exact location of thyroid follicles in the adult is variable. Perhaps every fish 
gives a recognizably distinct distribution pattern. To understand the location of the 
thyroid some consideration of the anatomy of the skeletal and other parts of the floor 
of the mouth is necessary. There are five pairs of branchial arches, of which only the 
cephalic four bear gills. Each arch save the fifth is composed of several bones, of 
which the ventral, or hypobranchial, is united with its fellow of the opposite side by the 
mesal unpaired basibranchial, and by cartilaginous copulae which thus form the links 
