One of several paired grooves in the embryonic endoderm, lateral to the corresponding pharyngeal pouch in the ectoderm.
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)