Embryogenesis

Abstract Embryogenesis is the process in which a single cell develops into a multi-cellular organism. This process varies widely among different organisms, but some overarching features are conserved. The first stage is fertilization, where a male sperm meets a female ovum and the two cell fuse to form a zygote. The zygote then divides in a process known as cleavage. The ball of cells formed in the cleavage process, the blastula, transforms through the process of gastrulation, during which the embryonic axes are defined and the germ layers are determined. After gastrulation, there is a long and intricate process of organogenesis, during which the final structure of the body is assembled. The variability in the developmental process can have phylogenetic significance. A large super-phyletic clade of animals, Spiralia, is defined by the presence of spiral cleavage. The blastopore, the first opening through which gastrulation occurs, develops into the mouth in the clade known as Protostomia, and into the anus in the clade known as Deuterostomia.