Invagination means to fold inward or to sheath. In biology, this can refer to a number of processes. Invagination's the morphogenetic processes by which an embryo takes form, and's 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.… (More on Invagination)