the folding in of an outer layer so as to form a pocket in the surface
<noun.process> the invagination of the blastula
Invagination \In*vag`i*na"tion\, n. [L. pref. in- + vagina sheath.] 1. (Biol.) The condition of an invaginated organ or part.
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]
Note: 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.