invagination

In*vag`i*na"tion, n. Etym: [L. pref. in- + vagina sheath.] 1. (Biol.) Defn: The condition of an invaginated organ or part. 2. (Biol.) Defn: 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.

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