Source : Webster's Revised Unabridged Dictionary (1913)

Gastrulation \Gas`tru*la"tion\ (g[a^]s`tr[.u]*l[=a]"sh[u^]n), n.
   (Biol.)
   The process of invagination, in embryonic development, by
   which a gastrula is formed.

Source : WordNet®

gastrulation
     n : the process in which a gastrula develops from a blastula by
         the inward migration of cells