The peripheral endocrine tissues

Abstract

Throughout the vertebrates the thyroid is a derivative of the mid-ventral floor of the embryonic pharynx. The cyclostome pattern differs from that of the gnathostomes only in that the thyroid tissue develops along almost the entire length of the pharynx rather than from a relatively short anterior region. In the earliest myxinoid embryos that have been examined, the thyroid appears to have arisen through the closing off of a ventral pharyngeal groove and not from an endostyle-like structure of the ammocoete type. From this embryonic rudiment, a continuous chain of cell cords is produced, passing upwards towards the oesophagus in the adipose tissue surrounding the gill pouches. These cell groups, at first solid, eventually hollow out to form thyroid follicles (Fernholm, 1969). In lampreys, the definitive thyroid tissue does not appear until metamorphosis, when it develops from certain epithelial elements that appear to survive the otherwise complete breakdown of the larval endostyle.