THYBEOID GLAND OF THE TELEOSTS 



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Fig. 21. Sections through the thyreoid gland of Pleuronectes. A, in the aortic 

 bifurcation; B, just posterior to it. 



The first branchial arteries are partly surrounded by follicles. 

 Behind the aortic bifurcation there is an open space for lateral 

 extension, but not for ventral, since the aorta rests on the muscula- 

 ture. The caudal end of the gland lies a little behind the second 

 branchial arteries, and consists again of small scattered follicles 

 (fig. 20,(7). 



The follicles are of globular or long ovoid shape, some are very 

 irregular. The circular cross sections vary from 20 to 500^ in 

 diameter. The very large ones lie at the second arterial branches. 

 Branched follicles with connecting channels between them are 

 numerous, so that almost all follicles may be traced in sections 

 as evaginations of others. 



The follicular epithelial cells vary from highly cylindrical, 

 narrow shapes to broad cuboidal. Flattened epithelium is rare. 

 The cells are extremely numerous and densely arranged. The 

 nuclei are located near the base of the cells, even in the higher ones, 

 and the cytoplasm stains darker about the nucleus. Sometimes 

 it appears as if there were a cuticle on top of the cells, as many 

 authors have described. This, however, is nothing else than a 

 refractive appearance of the cell margin from which the cytoplasm 

 has slightly retracted. The blood supply is extremely rich. 



(A parasitic worm was found in this thyreoid and had caused a 

 considerable hemorrhage.) 



