Nearly all animals undergo some form of sexual reproduction.[16] They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.[17] These fuse to form zygotes, which develop into new individuals.[18] Many animals are also capable of asexual reproduction.[19] This may take place through parthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation.[20] A zygote initially develops into a hollow sphere, called a blastula,[21] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.[22] In most other groups, the blastula undergoes more complicated rearrangement.[23] It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers Ч an external ectoderm and an internal endoderm.[24] In most cases, a mesoderm also develops between them.[25] These germ layers then differentiate to form tissues and organs The blastula (from Greek ??????? (blastos), meaning sprout) is a hollow sphere of cells formed during an early stage of embryonic development in animals.[1] The blastula is created when the zygote undergoes the cell division process known as cleavage.[2] The blastula is preceded by the morula and is followed by the gastrula in the developmental sequence.[citation needed] A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds an interior central cavity known as the blastocoel.[3] [4] In mammals, blastulation leads to the formation of the blastocyst, instead of the blastula. The blastocyst contains an embryoblast (or inner cell mass) that will eventually give rise to the definitive structures of the fetus, and the trophoblast, which goes on to form the extraembryonic tissues