THYREOID GLAND OF THE TELEOSTS 755 



not all the nuclei of a follicle show the same structure or the same 

 reaction towards the stain and thus may be in different physi- 

 ological stages. In addition to normal, large nuclei with distinct 

 nucleoli and granular structure we find compact deeply staining 

 nuclei which sometimes contain a vesicle. There are also small 

 pyknotic nuclei in small (degenerating) cells. Often such com- 

 pact nuclei with a halo of colloid are found within the lumen and 

 it seems then that the epithelial cells have emptied their entire 

 content. These masses can be easily distinguished in the colloid 

 even after their outlines become indistinct as they have a differ- 

 ent refractive index. Maurer describes somewhat similar struc- 

 tures in trout and carp. In other cases several neighboring cells 

 with much swollen bodies have been pushed off from the epi- 

 thelium and may be seen in the colloidal substance (pi. V, fig. 

 13). 



The general form of the follicles is globular, though the sur- 

 rounding fat and muscle tissue influences the outlines to some 

 degree (pi. V, figs. 10-12). 



Smooth muscle fibres are found in the entire thyreoid region; 

 in one case (the first specimen) only ventral to the aorta. They 

 run in all directions in the interfollicular tissue. The follicles are 

 often arranged along them or are surrounded by them. Where 

 the follicles lie in clusters of five or ten or more, smooth muscle 

 fibres are found running between them. The muscle fibres with 

 the follicles, their capillaries and the connective tissue fibres 

 form a somewhat compact structure. 



The blood supply to the secreting epithelium is extremely 

 rich, several capillaries going to each follicle (pi. V, figs. 10, 12 

 Ca). 



The thyreoid gland in two other species was dissected out as 

 far as it was visible macroscopically. In this way of course one 

 does not get the scattered follicles but only the main masses. 

 Figs. 28 and 29 of plate III from these two dissections as well 

 as figs. 25 to 27 of plate II, which are from specimens cut in 

 serial sections, show that the distribution of the thyreoid in the 

 trout is very variable. 



