This type of sexual reproduction occurs in homothallic species of Spirogyra. Here two cells of a filament take part in gametic union. First the protoplasmic contents these cells contract to form single gamete. The gamete from one cell (+ve) moves to the other cell (-ve) through a passage formed either in partition wall or across the partition wall of the two adjacent cells. The two gametes fuse inside the recipient (-ve) cell, forming a diploid zygospore.