THYREOID GLAND OF THE TELEOSTS 725 



follicles are closely arranged and so densely packed that apparently 

 only lymph spaces exist between them, in others we find the folli- 

 cles more loosely connected and suspended in the connective tis- 

 sue; while again in other specimens they lie so far apart that they 

 can scarcely be thought of as belonging to one organ. The his- 

 tological appearances also differ very much within the individual, 

 depending upon the region from which the section is taken. 



When the arrangement is such that the thyreoid may be dissected 

 out and then sectioned, the folhcles are found to be rather densely 

 packed (pi. V, fig. 17). By this method, however, we are unable 

 to get a correct idea of the extension of the thyreoid and the ar- 

 rangement of its folhcles, since it is only possible to remove the 

 somewhat denser masses around the stem of the aorta, usually near 

 the base of the second aortic arch, and all the particles in front 

 and behind this region still remain. Properly to study the general 

 distribution of the thyreoid folhcles serial sections through the 

 entire gill region are absolutely essential. 



The spaces between the muscles, branchial arteries and gill 

 arches are filled by wide-meshed connective and fatty tissue. In 

 these tissues the follicles are suspended. The connective tissue 

 is, therefore, not so directly a part of the thjTeoid organ in these 

 fish as it is in the encapsuled organ of mammals. The primary 

 object of this tissue is to form a connection between the muscles 

 and bones without regard to whether there may be thyreoid 

 tissue in the region or not. True interstitial tissue, as such, is 

 not found in this diffusely scattered thyreoid organ. Of course, 

 the tissue in which the follicles lie imbedded performs the same 

 function as does the capsule in higher vertebrates: in both cases 

 it serves to support the folhcles. In glands, where many follicles 

 are accumulated in one mass, as in Cynoscion, or in the central 

 portions of some others, for instance the trout, the supporting 

 tissue may be regarded as part of those masses, but not as part of 

 the entire thyreoid gland; here also the formation of connective 

 tissue is the primary process, and the suspending of the follicles 

 only a secondary one. 



The supporting tissue is simple except in two species, Salveli- 

 nus and Sarda (pi. V, fig. 21) where smooth muscle fibres are freely 



JOURNAL OF MORPHOLOGY. VOL. 21, NO. 4 



