What is the meaning of Gastrula?

A stage in the development of embryos of most animals consisting of a three-layered sac of ectoderm, mesoderm and endoderm.

gastrula

third-person singular past historic of gastruler

gastrula (stage in the development of embryos of most animals consisting of a three-layered sac of ectoderm, mesoderm, and endoderm)

Source: wiktionary.org