What is invagination?
invagination meaning an inward folding of a layer of cells forming an interior pocket
The cells at the vegetal hemisphere of this starfish blastula have begun to push into the blastocoel (invaginaton), forming a diploblastc embryo with a new cavity, the primitive gut, or archenteron. The cells lining the archenteron are endodermal cells. (Photo: Dr. Anna E. Ross, Christian Brothers University, TN)
reference: Coral Reef Information System – GlossaryWhat is