THYREOID GLAND OF THE TELEOSTS 745 



apart, carrying the follicles with them. The cells, which were 

 in contact with others or with blood and lymph vessels may have 

 held fast to them, becoming drawn out into long processes. 

 They thus form a network between the follicles. These bridges 

 often surround the capillaries. 



There are only a few follicles which have a regular epithelium 

 with a smooth outline. Outside the bony ring, described above, 

 the follicles have the usual epithelium with a smooth surface. 



In places the epithelium was found to be disintegrating. The 

 association of the cells seemed rather loose, their surfaces were 

 also drawn out into long processes like pseudopodia which some- 

 times divided into two and disappeared in the interfollicular 

 tissue (pi. IV, fig. 5, E). These cells did not show any distinc- 

 tion between nucleus and cytoplasm, and their contents was of 

 a foamy nature and showed two or three com^pact deeply staining 

 granules. They were probably cells which having completed 

 their secretion period were disintegrating. 



SALMO IRIDEUS GIBBONS 



Specimens 4 cm. long, one month old. In the young rainbow- 

 trout the thyreoid gland begins in the aortic bifurcation and 

 extends almost to the third gill arteries, (pi. II, fig. 22). There 

 is little space for a lateral extension, as the cartilages of the basi- 

 and hypohyalia form a rather narrow arch, and limit the gland to 

 the space immediately above and below the aorta. At the aortic 

 bifurcation the copula comes close to the vessel, so that the fol- 

 licles are pressed away from the median line, and lie close to the 

 sides of the cartilage. Later the skeletal parts move back, the 

 space between them becoming somewhat clearer. Half-way 

 between the first and second gill branches the thyreoid gland 

 also extends below the aorta, and a large number of the follicles 

 lie near the second arterial branches. These ventral follicles 

 are smaller than the dorsal ones (fig. 4, B). Towards the pos- 

 terior limit the follicles become smaller and fewer and are again 

 limited to the region above the aorta. Only two or three folli- 

 cles are seen in a cross section and at the third gill arteries 

 they have entirely disappeared. 



