any structural anomaly of the outpocketing of the caudal-most branchial pouch of the embryo (interpreted as a fifth pouch in human or as the ventral component of the fourth pouch in mouse) that fuses with the thyroid diverticulum, ultimately giving rise to calcitonin-producing parafollicular cells (aka C-cells) which function in calcium homeostasis; in most mammals, these parafollicular cells fully disperse with the endodermally derived follicular cells of the thyroid by birth