invaginate

adj

1. (not-comparable)sheathed
2. (not-comparable)Having one portion of a hollow organ drawn back within another portion.

verb

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