First Stage of Invagination
The large endodermal cells that are remaining in the vegetal plate move laterally towards the centre of the plate and fill the gap in the vegetal plate caused through the ingression of the primary mesenchyme cell. As a result, the vegetal plate turns into even more flattened. Soon thereafter the plate bends inwards (invagination) in the centre initiating the creation of a new cavity called the archenteron (primitive gut). Its opening at the vegetal pole is the blastopore. Invagination proceeds till the archenteron extends into the blastocoel about one third the distances among the vegetal and animal poles and then stops. The endodermal cells invaginate even if the vegetal plate is isolated and cultured in vitro pointing out that invagination occurs due to intrinsic properties of these cells and no external force is involved. Invagination possibly occurs as a result of constrictive activity of microfilaments at the outer ends of the cells of the vegetal plate.