720 J. F. GUDERNATSCH 



During development of the Teleosts some follicles cling to the 

 wall of the aorta and are in later life found along it. Usually 

 the follicles become arranged into several distinct groups, forming 

 different centers of growth, as is shown by Cynoscion in plate 

 III. With the branching off of the gill arteries from the aorta 

 thyreoid material is carried out laterally towards the gills and 

 spreads in this region. This accounts for the larger lateral exten- 

 sion of thyreoid follicles along the gill vessels rather than in inter- 

 mediate regions. The larger vessels form a substratum upon which 

 the follicles migrate as do also the smaller vessels and especially 

 the lymphatic vessels. The larger vessels are means for the an- 

 tero-posterior dispersion while the smaller ones allow the migra- 

 tion of follicles from the denser central thyreoid portions towards 

 the periphery. Even the most peripheral follicles are usually 

 found near blood capillaries although they do not necessarily 

 come in close contact with them. The way in which these isolated 

 follicles function is not clear. They certainly seem normal and 

 contain colloid. 



The growth of the connective tissue and fat in which the foUi- 

 cles are imbedded favors their dispersion from the central portions ; 

 thus a combination of influences are at work to widen the thyreoid 

 region as much as possible. W. Miiller regards the immense 

 development of the 'interstitial' tissue as alone responsible for 

 the dissolution of the thyreoid into isolated groups. He no doubt 

 refers to fat and connective tissue, as we shall see below that the 

 term 'interstitial' is not properly used in this case. Although 

 the growth of these tissues may be an important factor T do not 

 regard it as primary, since in the first place, even in young indi- 

 viduals, the follicles are found isolated, and secondly, the breaking 

 apart of a formerly compact organ by excessive growth of connec- 

 tive tissue would certainly not account for the carrying of the 

 follicles into the muscles and gills. 



The follicles actually seem to overcome the obstruction offered 

 by other tissues in their course and may even penetrate into them. 

 In trout and Micropogon the thyreoid follicles are at times im- 

 bedded in the muscle tissue, into which they creep between the 

 connective tissue lamellae or along the blood vessels. Realactiv- 



