744 J. F. GUDERNATSCH 



The parts of the basibranchiale and hypobranchiale are not 

 compactly developed, the floor of the pharynx being supported 

 by a scaffold of osseous lamellae, between which a wide-meshed 

 fatty tissue appears. There is also an osseous tube open at both 

 ends (representing perhaps a sub-copula) and enclosing the most 

 anterior portion of the ventral aorta (fig. 3, A). In this tube the 

 thyreoid gland extends from the branchial vessels towards the 

 tip of the tongue. In transverse section the gland appears to be 

 lying within a bony ring which completely separates it from the 

 parts outside. At certain places, however, there are openings in 

 this capsule through which the folHcles escape into the outside 

 tissue. Within the capsule are found osseous lamellae dividing 

 it into several compartments, and thus three or more bunches 

 of thyreoid tissue may be seen separated by bone. 



At the anterior end the follicles lie outside the osseous cap- 

 sule and are scattered far apart. They are always located in 

 the neighborhood of either blood or lymph vessels and probably 

 follow the vessels as paths of dispersion. The follicles are not 

 always, however, in direct contact with blood vessels. 



The osseous capsule lies above the ventral aorta, and we find 

 thyreoid tissue only above the vessel. The first gill arteries 

 for a short distance are completely surrounded by very small 

 follicles. From the aortic bifurcation the follicles extend far 

 forward into the capsule although there are no large vessels, 

 thus there seems to be a tendency towards a forward migration. 

 This is really the only available space into which the thyreoid 

 can expand, unless it enter the ventral musculature. 



The histology of the gland is somewhat different from that in 

 other fish. The follicular epithelial cells are drawn out into long 

 processes which come into contact with those arising from the 

 cells of near-by folUcles (pi. IV, fig. 1). This suggests that the 

 cells of one follicle might communicate through these processes 

 with those of the adjacent follicles. The only explanation for 

 this phenomenon is as follows: originally the follicles lie close 

 together, with their epithelial cells touching, and when the space 

 between the skeletal parts becomes wider the meshes of the fatty 

 tissue, in which the follicles are suspended, are pulled somewhat 



