ingression

noun3

1. •
   The act or process of entering or intruding.
2. •
   The inward migration of cells from the blastula during gastrulation.

   • Ingression of ectodermal cells from the area between the velar lobes establishes the cerebral ganglia and similar ingressions produce the other ganglia. — 1940
3. •
   The process by which a potentiality enters into actuality.

   • […] it is by virtue of the individual essence that an eternal object remains precisely the same eternal object through all its ingressions […] — 1967