Residuals of the Third Branchial Pouch Origin
Single-cell-layered vesicles, which are almost similar in appearance to those derived from the peripheral part of the ultimobranchial body, are also produced from the third branchial pouch. They are found just in the neighborhood of the isthmus but not included within the isthmus or in the caudal extremity of the thyroid lobe. The third branchial pouch origin is more exactly confirmed by remaining combined directly with the parathyroid (III) gland (Fig. 25b). The other single-cell-layered vesicles which are near the isthmus and not combined with the parathyroid (III) gland are indistinguishable from those of ultimobranchial origin.