THYREOID GLAND OF THE TELEOSTS 713 



derivates are still easily modifiable and do not yet represent a per- 

 manent condition. 



The thyreoid gland of Teleosts is not a single compact organ, 

 as we find it in the higher vertebrates, where the small parts of the 

 gland, the follicles, are united into one complex and enclosed by a 

 common capsule of connective tissue. Only in such a case would 

 the term 'gland' be justified, since here numerous anatomical 

 elements possessing the same physiological function are closely 

 connected. The Teleosts, however, possess numerous elements, 

 whose totalitj^ from a physiological standpoint one must regard 

 as a thyreoid gland, while anatomically we are unable sharply 

 to define the organ in question in this group of vertebrates. In 

 most cases we can speak of thyreoid follicles only, or groups of 

 follicles, in pointing out the distribution of the organ. Thus in 

 plates I and II, not the thyreoid glands of the respective species, 

 but the regions of distribution of the thyreoid follicles themselves 

 are figured. On plate III, however, which indicates the variation 

 in the location of the gland within the species, real parts of the 

 thyreoid are indicated, so far as they were macroscopically visible. 



In the more closely defined region we may find thyreoid tissue 

 in all parts. It is usually of a brownish yellow color. The follicles 

 are generally most densely located in the neighborhood of the 

 ventral aorta or of the branchial arteries. Those places, particu- 

 larly, are favored where the branchial arches arise from the aortic 

 stem. Follicles are most abundant at the origin of the second 

 gill arteries, that of the first being next, and finally the roots of 

 the last branches have fewest thyreoid follicles about them. A 

 more or less dense accumulation is always found along the stem 

 of the ventral aorta, which may be completely surrounded by 

 thyreoid tissue. In other cases dense accumulations of follicles 

 are located either dorsally or ventrally to the aorta. 



The glandular tissue is usually distributed so that it is more 

 densely accumulated in the central region, while towards the pe- 

 riphery a more and more pronounced dissolution and scattering of 

 the follicles takes place. These conditions cannot, however, be 

 strictly generalized, as we find cases in which, even in the most 

 central portions, the follicles are not more closely arranged than 



