What does "invagination" mean?

Invagination: (Biol.) One of the methods by which the various germinal layers of the ovum are differentiated. 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.