invaginate

Short definition from Wiktionary

Adjective

1. 1
   sheathed
2. 2
   Having one portion of a hollow organ drawn back within another portion.

Verb

1. 1
   To fold up or enclose into a sheath-like or pouch-like structure, either naturally or as part of a surgical procedure.
2. 2
   To turn or fold inwardly.
3. 3
   To fold inward to create a hollow space where none had existed, as with a gastrula forming from a blastula.