A diverticulum from the fourth pharyngeal pouch of an embryo, regarded by some as a rudimentary fifth pharyngeal pouch and by others as a lateral thyroid primordium. The ultimobranchial bodies of lower vertebrates contain large amounts of calcitonin. In mammals the bodies fuse with the thyroid gland and are thought to develop into the parafollicular cells. (Stedman, 25th ed)
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