374 BULLETIN OF THE BUREAU OF FISHERIES. 



representative. In 1911 Gudernatsch, 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 fonfanalis}. He was the first to emphasize, in a preliminary 

 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 with 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 Salmonidse. 

 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 Gudernatsch (1911, a, p. 753 and pi. n) 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 hvoid arteries, and consists 



