Multicellular organisms reproduce during the process of meiosis. During meiosis, haploid gametes are produced and then fertilized to form a diploid zygote. Meiosis and fertilization cause variation into offspring that contributes to help organisms reproduce for the next generation. The great majority of eukaryotic organisms must go through meiosis and fertilization to reproduce. Multicellular organisms can also reproduce asexually. Many multicellular plants reproduce by asexual spores, which can be aerial or motile and aquatic. Whole portions of the vegetative part of the organism can bud off and begin the process of creating a new individual. Various species of sponges also reproduce asexually, and produce gemmules that may later become new sponges.