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BULLETIN OF THE BUREAU OF FISHERIES. 
one might expect to find instances of remotely placed deposits. We are therefore 
surprised to find the sharp delimitation to the region already described which appears to 
exist in our wild specimens, with one marked exception, i. e., the jugular pit to which 
reference has already been made. It has been assumed by some writers that aberrant 
thyroid deposits in some regions might be frequently expected. This has been due to 
the development of tumor nodules in regions beyond the normal and usual seat of 
distribution, such as the lower mouth parts and gill arch region. The only outlying 
tumors of this sort which may be inferred with certainty to arise from original deposits 
of thyroid are the so-called pit tumors. 
In the adult trout there occurs on the ventral side of the head between the dentary 
bones a median irregular depression or blind pit open to the exterior. It is the region 
where the muscles between the dentaries and those of the branchiostegals become 
common. The skin dips into its ramifications with many plications and infoldings. 
It is an entirely superficial and exterior landmark, and though independent of and well 
removed from the thyroid region in the adult, is of some importance in thyroid pathology, 
since at an early stage it is in closely apposed relation to the thyroid region, and is the 
occasional seat of detached thyroid follicles and of independent thyroid tumors. This 
much branched and partitioned cavity may be designated as the jugular pit (fig. ii). 
It may be recognized at or soon after the hatching of the embryo. Normal thyroid 
has been several times observed immediately beneath the pit epithelium in wild brook 
trout (fig. 14), where its presence may be readily explained by the mechanics of devel- 
opment of the parts (fig. 12). Since of 91 trout with tumors, 25 showed pit tumors, one 
may conclude that more than one-quarter of all brook trout have normally some thyroid 
follicles in the region of the pit. 
Occasionally detached tumors develop in the gill arches. We have never actually 
seen thyroid follicles on the free portions of the gill arches, and there is little embryo- 
logical presumption in favor of such deposits, notwithstanding the relation of the gill 
arches with the thyroid region. The tip of the lower jaw has even much less anatomical 
relation to the thyroid region, rendering it an unlikely place for stray follicles. The 
examination by serial sections of the tips of the lower jaws of about 25 trout failed to 
show any thyroid here. The region is called into question as a seat of normal thyroid 
deposit on account of the rare cases of a thyroid tumor occupying this site. The 
actually observed occurrence of normal thyroid in the pit and the incidence of tumors 
here make it extremely probable that no other extralimital deposits occur with any 
frequency, else such would declare themselves in tumor formation. 
HISTOLOGY. 
A closed alveolus is the unit of the thyroid gland. These alveoli are apparently 
independent of each other save as they are more or less bound together by the vascular 
framework and connective tissue stroma. Their lumina have no connections with 
each other and each alveolus independently discharges its secretion into the circulation. 
Many alveoli are quite isolated from their kind and lie loosely in the tissue, whether 
connective, muscle, bone, cartilage, or fat. In the simplest adult condition (fig. 14-21) 
