Abstract
A histological examination of the thyroid glands of the threespine stickleback was carried out through various stages from the fry just hatched to the young kept for 110 days.The glands were highly active as judged from the histological criterion in the juveniles kept for about 20 days in freshwater.The juveniles survived in sea water began to have deposited guanine in the skin.In freshwater, the epithelial wall of the thyroid in the stickleback at the age of one month was irregular in shape and folded up irregularly.Then the fish which were transferred into sea water developed a heavy coat of guanine crystals.The thyroid follicles of the specimens kept continuously in freshwater beyond one month or more were excessively enlarged, and the interstitial connective tissue had markedly decreased in amount.Nucleus -like bodies and colloid-like droplets were often found in the granulated and eosinophilic colloid of the follicular lumen.The colloid-like droplets were stained pink with eosin and orange or brownish orange with Mallory's triple stain.Although small follicles were scattered in the branchial arch, the larger follicles were united with one another.
When the fish were transferred into sea water, a spontaneous involution of the enlarged thyroid occured.
