Delamination

Delamination \De*lam`i*na"tion\, n. (Biol.)
   Formation and separation of lamin[ae] or layers; one of the
   methods by which the various blastodermic layers of the ovum
   are differentiated.
   [1913 Webster]

   Note: This process consists of a concentric splitting of the
         cells of the blastosphere into an outer layer
         (epiblast) and an inner layer (hypoblast). By the
         perforation of the resultant two-walled vesicle, a
         gastrula results similar to that formed by the process
         of invagination.
         [1913 Webster]