Invagination (?), n. [L. pref. in- + vagina sheath.]
[1913 Webster]

1. (Biol.) The condition of an invaginated organ or part.
[1913 Webster]

2. The inward movement of the wall of a tissue or cell, to form a cavity; also, the cavity thus formed.
[PJC]

3. Specifically: (Biol., Embryology) The inward movement of one part of the wall of a blastula, to form a gastrula; the process of gastrulation, in which layers of the ovum are differentiated.
[1913 Webster +PJC]

&hand_; In embolic invagination, one half of the blastosphere is pushed in towards the other half, producing an embryonic form known as a gastrula. -- In epibolic invagination, a phenomenon in the development of some invertebrate ova, the epiblast appears to grow over or around the hypoblast.
[1913 Webster]