The developing embryo or embryogenesis is the process by which the embryo is formed and developed. In mammals, the term primarily refers to the early stages of prenatal development, while the terms fetus and fetal development describe later stages.
Embryogenesis begins with the fertilization of the egg (ovum) by a sperm cell (sperm). Once fertilized, the egg is known as a zygote, a single diploid cell. The zygote undergoes mitotic divisions without significant growth (a process known as cleavage) and cell differentiation, leading to the development of a multicellular embryo.
Although embryogenesis occurs in both animal and plant development, this article addresses the common characteristics between different animals, with some emphasis on the embryonic development of vertebrates and mammals.
The ovum is generally asymmetric, with an "animal pole" (future ectoderm and mesoderm) and a "plant pole" (future endoderm). It is covered with protective envelopes, with different layers. The first envelope - the one that is in contact with the egg membrane - is made of glycoproteins and is known as the vitelline membrane (zona pellucida in mammals). Different taxa show different cell and acellular envelopes encompassing the yolk membrane.
Fertilization (also known as 'conception', 'fertilization' and 'syngamy') is the fusion of gametes to produce a new organism. In animals, the process involves a sperm that fuses with an egg, eventually leading to the development of an embryo. Depending on the animal species, the process can occur inside the female's body in internal fertilization, or outside in the case of external fertilization. The fertilized egg is known as the zygote.
At some point after the different layers of the embryo are defined, organogenesis begins. The first stage in vertebrates is called neurulation, where the neural plate folds to form the neural tube (see above). Other common organs or structures emerging at this time include the heart and somites (also above), but as of now embryogenesis does not follow a common pattern among the different taxa of the animal kingdom.
In most animals organogenesis along with morphogenesis will result in a larva. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.
