Thymus and inferior parathyroids usually develop from the third pharyngeal pouch around the sixth foetal week. The inferior parathyroids separate from the thymic tissue and remain close to lower pole of the thyroid, while the thymus descends into the mediastinum. During thymic migration, small fragments of thymus may separate and attach themselves to any site along this route. Parathyroid tissue may be seen close to or embedded within the thymus as a result of their common origin and path of descent. Third branchial apparatus anomalies are rare and constitute less than 1% of all cases of branchial anomalies cases.
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