Several models of primary invagination have been proposed. These include:
(1) apical constriction - in this model, which has been proposed for many decades, the apical ends of some cells in the vegetal plate would actively constrict, forcing the vegetal plate to buckle.
(2) apical tractoring (Burke et al., 1991. Dev. Biol. 146,542-57) - in this model, cells lateral to the vegetal plate actively migrate towards the vegetal pole, forcing the vegetal plate to indent. In this model apical surfaces of the lateral cells would have to migrate actively.
(3) swelling of proteoglycan (Lane et al., 1993. Development 117, 1049-60) - in this model, local secretion of chondroitin sulfate proteoglycan would result in invagination if after its release the proteoglycan swells, and the swollen material is held in place by the hyaline layer.