Histochemical distribution of peroxidase in amphioxus and cyclostomes with special reference to the endostyle

The histochemical distribution of peroxidase was studied in amphioxus, ammocoetes larvae, adult lampreys, and hagfish. The endostyle of amphioxus displayed peroxidase activity in zone 5 and, in some individuals, in zone 1 as well. The endostyle of ammocoetes exhibited strong peroxidase activity in type 2c and type 3 cells. These peroxidase-positive regions coincide well with radioiodine-binding regions previously described. Thyroid follicles of adult lampreys stained strongly for peroxidase, but those of the hagfish did not. The branchial sac of amphioxus showed peroxidase activity, but the gill sac of cyclostomes did not. The intestine did not show peroxidase activity in amphioxus or in cyclostomes.