374 
BULLETIN OF THE BUREAU OF FISHERIES. 
representative. In 1911 Gudematsch, in the most extended study of the fish thyroid 
that has yet appeared, diagramed the distribution of the follicles in 22 genera of teleosts 
including 4 genera and 5 species of salmonoids, and showed the minute structure in 
several genera including 2 species of salmonoids, viz., a Pacific salmon and the American 
brook trout (Salvelinus fontinalis). He was the first to emphasize, in a preliminaiy' 
statement before the American Society for Cancer Research (Nov. 27, 1909), the lack of 
a capsule in the thyroid of teleosts. 
It is thus seen that a number of studies having to do wdth normal fish thyroid have 
been made, and are widely scattered among the many and diverse genera of this great 
class. Not until recently has any particular attention been focused upon the Salmonidae. 
The amount, distribution, and structure of the gland may be said to have been shown for 
individuals whose source and history and the conditions under which they had been 
living are not well known or are not stated, but which are presumptively normal and 
show no obvious pathologic changes. If, however, one limits the normal to the minimum 
of the thyroid exhibited by adults from streams far from and unaffected by civilization, 
where the fish are obviously living strictly in a state of nature, there is yet but a meager 
exposition of the normal thyroid in the salmonoids. We believe that the final comparison 
is to be against a norm set up by such individuals, and that most trout from aquariums, 
markets, fish-cultural establishments, and from artificially stocked streams and lakes or 
unstocked streams or lakes close to civilization or much frequented by people, have 
either abnormal thyroids or are not to be judged by criteria obtained from strictly 
wild trout. 
In our specimens of wild brook trout we are unable to find the thyroid distributed 
as widely and in such quantity as shown by Gudematsch (1911, a, p. 753 and pi. ii) for 
this species. He finds it extending into the gill arches, infiltrating muscle bundles and 
in places completely filling the available thyroid spaces. We find these conditions in 
domesticated fish, but not in our wild specimens. His material was in part obtained from 
aquarium fish, and in such we would expect such a distribution. It may even occur 
in specimens from some streams or lakes. We would infer that all fish exhibiting it 
may be presumed to have been under influences foreign to those usually obtaining in 
strictly wild natural conditions, but they may perhaps be considered to represent a 
normal for trout under a modified regime without presumption of any definite pathologic 
change. The minimum quantity of thyroid and its more restricted distribution appear 
to us as affording a more representative picture of the ultimate normal. Maurer, while 
not mapping in detail the distribution in the adult, describes a condition which speaks 
for the more confined arrangement of thyroid tissue (fig. 5 and 6). 
EMBRYOLOGY. 
Maurer (1886) has described the development of the thyroid in trout. According 
to his observations, about the twenty-seventh day after fertilization, the embryo being 
6 millimeters long, an unpaired median evagination arises from the ventral epithelium 
of the pharynx (fig. 7). This is the earliest differentiation of thyroid. It lies in front 
of the heart in the bifurcation of the heart tube into the hyoid arteries, and consists 
