Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. (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.

Source : WordNet®

invagination
     n 1: the condition of being folded inward or sheathed [syn: {introversion}]
     2: the folding in of an outer layer so as to form a pocket in
        the surface; "the invagination of the blastula" [syn: {introversion},
         {intussusception}, {infolding}]