Reproduction occurs by both asexual and sexual means. During the asexual reproduction cells of the lower half of the colony enlarge in size, possess prominent nucleus, dense cytoplasm and form the gonidia. It then increases in size and divides longitudinally and forms the 8 celled stages called Plakea. It then divides further and form a 16 celled stage which arrange themselves to form a hollow sphere called Phialophore. After reaching a definite number in the coenobium the colony curved inwards at the centre by the invagination through the Phialophore region. Then the flagella and the eye spot develop and form the parent colony.
Sexual reproduction: Oogamous type of sexual reproduction is common in the volvox. Where the antheridia are the male sex organ and oogonia is the female sex organ. During sexual reproduction some of the cells of the posterior half of the coenobium withdraw their flagella, enlarge and become gametangia. The male gametangium gives rise to antheridium and the female gametangium to oogonium. Antheridia are produced in large number. The protoplasm of the gametangia divides by longitudinal division resulting in the 128 male cells depending on species. The protoplasmic piece differentiated to form the motile antherozoids. The female gametangia enlarges in size, rounded and flask shaped. The protoplasm then develops into a spherical egg or oosphere. The mature oogonium produces some chemical substance which attracts the antherozoids. The antherozoid reaches the egg and result in the fertilization and formation of the zygote. Zygote secretes its cell wall which is three layered made of exospore, mesospore and endospore. Zygote then undergoes meiosis forms four haploid nucleus of which only one survives. The functional nucleus metamorphoses into meizoospore. It then divides successively and simultaneously to form the hollow sphere and atlast produces the entire vegetative colony. The diploid phase is restricted to the zygote alone; the dominant phase in the lifecycle is the haploid.
