It is very interesting to find this reproductive phase in Spirogyra. The sexual reproduction takes place by conjugation in Spirogyra. Zygospore is the final product of conjugation. In this video you can see the lateral conjugation. Between two conjugating cells, conjugation tube is formed. After conjugation tube formation, the protoplasmic mass of the male cell rounded to form male gamete and protoplasmic mass of female cell rounded to form female gamete. The male is active donor. It moves towards the female through conjugation tube. Fusion product of male and female cell is the zygospore. It is diploid, thick walled structure.