THYREOID GLAND OF THE TELEOSTS 733 



network of colloidal threads is shown between them. In higher 

 vertebrates these large surface irregularities in the colloid seem 

 more common than in fish. The second, smaller irregularities 

 might resemble secreted droplets. They give to the surface of 

 the colloid, especially from a top view, the appearance of being 

 beset with oil drops. In some places there are merely slight depres- 

 sions in the free margin, some distance apart, while in others 

 the whole surface is corrugated, but these irregularities do not 

 appear in all of the follicles. Whether they are really physio- 

 logical products of the cells is not determined. The irregulari- 

 ties may be more easily explained on the theory that where the 

 free ends of the cells do not come in close contact, the colloid 

 which fills the follicles is pressed into the intercellular spaces 

 and surrounds the top of the cells like a cap. In shrinkage from 

 fixation the caps would be pulled from the cells, leaving on the 

 surface of the colloid the impressions. Anderson regards these 

 'droplets' as well as the numerous vacuoles, which he finds within 

 the colloid even of living glands, as "cavities lined with a hyaline 

 membrane and containing the 'chromophobe' secretion, a part 

 of the secretory activity of the gland." Langendorff and others 

 more correctly regard them as artefacts, having no physiolog- 

 ical significance. Vacuoles within the colloidal substance are 

 seldom seen, (pi. V, fig. 13, V). 



The colloidal material seems to become denser with age, as 

 far as this can be determined by its staining capacity. In young 

 trout it is rather pink so that it can scarcely be distinguished from 

 the blood serum in the vessels. Both structures show the same 

 microscopic appearance. In older trout, however, the colloid 

 stains very deeply with acid dyes. These observations agree 

 with those of Schmid on dogs of different ages. Anderson, Boe- 

 chat, Peremeschko and others also state that the number of 

 folhcles containing a slightly staining finely granular colloid 

 diminishes with age, being small in old individuals. I failed to 

 find some follicles distinguished by a greater affinity for the 

 stain than others, as was claimed by Hiirthle, but did find that 

 sometimes within the same follicle the colloid stained differently 

 in different places. 



