Second Stage of Invagination
The 2nd stage of invagination starts with release of secondary mesenchyme from the tip of archenteron into the blastocoel. The cells of secondary mesenchyme stay in which they are released at the tip of archenteron but their filopodia extend up to and attach to the inner side of the wall of blastocoel opposite to blastopore. Contractions of these ' filopodia pull the archenteron in the direction of the animal pole causing further invagination of the endoderm. Finally, the archenteron bends and its tip fuse along with the blastocoel wall on the ventral side where the mouth opens later. The secondary mesenchyme then disperses in the blastocoel and gives increment to mesodermal organs.