Sea Urchin Development Biologists have learned a lot about fertilization and embryonic development by observing and experimenting with sea urchins. Sea urchin eggs are fertilized externally after the animals release their gametes into the water. Fertilization and subsequent development are similar to the changes that occur in the early embryos of mammals, but they are much easier to observe in sea urchin embryos. Sperm surround sea urchin eggs and lock onto egg membranes. Fusion of the membranes of a sperm and an egg triggers a cortical reaction. The vitelline layer quickly swells away from the egg membrane and creates a fertilization membrane that blocks entry of additional sperm. Fertilization combines the haploid sets of chromosomes of sperm and egg into a diploid zygote. Membrane fusion also activates the egg, triggering subsequent embryonic development. After fertilization, three processes establish an animal's basic body plan: cleavage, gastrulation, and organogenesis. Processes taking several hours have been speeded up in this time-lapse video. Cleavage divides the zygote into a ball of many smaller cells. The first division splits the zygote into two. Each of these cells divides, forming four cells. Each cell in the early embryo is called a blastomere. Division continues, forming a multicellular ball called a morula. Over several hours, cleavage eventually creates a hollow ball of cells called a blastula. The cavity inside the blastula is called the blastocoel.