Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation,[4] the massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism, with differentiated germ layers: endoderm, mesoderm, and ectoderm. More localized invaginations also occur later in embryonic development, The inner membrane of a mitochondrion invaginates to form cristae, thus providing a much greater surface area to accommodate the protein complexes and other participants that produce adenosine triphosphate (ATP).[5] Invagination occurs during endocytosis and exocytosis when a vesicle forms within the cell and the membrane closes around it.