delamination

n. (Biol.) Formation and separation of laminæ or layers; one of the methods by which the various blastodermic layers of the ovum are differentiated. [ 1913 Webster ]

☞ 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 ]