Invagination

Invagination means to fold inward or to sheath. In biology, this can refer to a number of processes.

*Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, 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, to form coelom, etc.

*Invagination is the formation of a cleavage furrow during cytokinesis in animal cells.

*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 ATP.

*Invagination occurs during endocytosis and exocytosis when a vesicle forms within the cell and the membrane closes around it.

In the humanities:

*Used to explain a special kind of meta narrative. Used by Rosalyn Krauss and Jacques Derrida ("The Law of Genre", Glyph 7 (1980).