tail bud

https://doi.org/10.1351/goldbook.11280

In the vertebrate embryo, mass of proliferating cells at the caudal end that arises from the primitive node.

Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 810 [Terms] [Paper]