THYREOID GLAND OF THE TELEOSTS 715 



of the follicles to some extent. The follicles apparently tend to 

 go around the basihyale and along its sides towards the tip of the 

 tongue. In the dogfish and shark where the hyoid region offers 

 a comparatively free space we fiid it occupied by the compact 

 thyreoid, which is pushed slightly forward of the aortic bifurcation 

 (Ferguson) . In these animals also the thyreoid is originally placed 

 in the bifurcation of the truncus; later, according to W. Muller, 

 the anlage moves forward and becomes encapsuled. In Raja, 

 on the other hand, it remains in the bifurcation. 



The dorso-ventral, in combination with the lateral, extension 

 of the thyreoid follicles seems to be more dependent upon the con- 

 figuration of the pharyngeal floor than does the cephalo-caudad 

 extension. In fishes, in which the isthmus region is deep and 

 narrow, as in Brevoortia, we find, as might be expected, the dorso- 

 ventral distribution of follicles far surpassing the lateral. While 

 in other species, for instance, Tautoga, in which the floor of the 

 pharynx is very broad, the lateral extension is the important one. 

 In general it may be said that the lateral outweighs the dorso- 

 ventral distribution of follicles. 



Dorsally the follicles are usually found between the ventral- 

 aorta and the copulae of the gill arches. In cases where those skele- 

 tal parts come close to the vessel the follicles are forced away 

 laterally, and sometimes intrude into the spaces between the copu- 

 lae and hypobranchialia. When the parts of the basibranchiale 

 lie well separated the follicles extend up between the copulae and 

 come to lie close to the mucous membrane of the pharyngeal 

 floor. 



The main mass of the organ lies almost exclusively above, or 

 dorsal, to the ventral aorta. This is opposed to Maurer's state- 

 ment of the case. Below the aorta is usually found the smaller 

 part of the gland and the follicles are also more loosely scattered. 

 The development of the thyreoid gland above the aorta should 

 be expected since there is usually much more open space between 

 the aorta and the gill arches than is found below the aorta where 

 the muscles lie close to the vessels. The thyreoid elements en- 

 deavor to intrude below the aorta as much as possible, and when 

 this vessel, in the region of the third branchial arteries, sinks deeper 



