Invagination, in biology, can refer to a number of processes.
One is one of 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 a differentiated endoderm, mesoderm and ectoderm. More localized invaginations also occur later in embryonic development.
Another is the formation of a cleavage furrow during cytokinesis in animal cells.
