Transition of the endostyle to the thyroid in lampreys and classical evolutionary hypothesis of the vertebrate thyroid gland. A Larval lampreys (lateral view) possess an exocrine endostyle (en), which transforms into thyroid follicles (thy) in the adult (ventral view). p, pharynx; ht, heart; or, orifice; bb, branchial basket; va, ventral aorta; hbm, hypobranchial muscle; tm, tongue muscle. Bars, 100 μm. B Classical evolutionary hypothesis of the relationship between the endostyle and the thyroid based on the previous studies. Although an endostyle-like thyroid primordium was observed in developing hagfish (Eptatretus stoutii) embryos in 1906, it has not been verified ever since . The primitive endostyle has been retained only in modern lampreys but was independently lost in the gnathostome and possibly in hagfish.