One of several paired evaginations in the embryonic endoderm that develop into epithelial tissues and organs such as the thymus and thyroid gland.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 791 (https://doi.org/10.1515/pac-2015-1202)