Ontogenesis is the individual development of an organism from its birth to death. The period of ontogenesis from the fertilized egg to the exit of the young individuals from egg shells or the body of the mother called embryonic (embryonic) development (embryogenesis). After the birth or hatching from the egg begins the post-embryonic period.
In the animal world are three common types of ontogenesis - hopper, nelechenaya and intrauterine. When the first of them is the development of the organism occurs with the metamorphosis, when the second formation of the embryo occurs in the egg, when the latter is inside the mother's body.
Embryogenesis of a person is divided into periods, each characterized by structural features, type of feeding, respiration and excretion. In medical practice usually use two concepts: for 8 weeks developing organism is called a fetus (see); starting with 9 weeks, i.e. since the formation of embryonic organs,- the fruit. The post-embryonic period of the person are divided into age periods. Cm. the Ontogenesis, Phylogenesis.

Ontogeny (from the Greek. ontos - existent genesis and development) - the history of individual development.
Charles Darwin gave evolutionary interpretation of ontogenesis. The similarity of embryos of different species he explained existing between the types of family relations. Further ontogenesis were investigated in various directions. Created the doctrine of embryonic sheets as a natural stage of development of all multicellular animals. Creation of experimental embryology gave the opportunity to explore the embryonic development of experimental way. In recent years successfully developed physiological and biochemical embryology. In the USSR began to develop ecological direction in embryology.
The embryonic variety of devices placed in several types of embryogenesis: free primary hopper, nelechenaya and secondary hopper. The latter consists of four forms: a) encapsulated; b) secondary free hopper; C) secondary hopper embryogenesis at birth; d) embryogenesis parasitic larva.
Type of embryogenesis finds in many cases, characterized by alternation in the course of evolution: from the original, royalty-free hopper to nelechennaya and next to one of the forms of secondary larval embryogenesis.
Type of embryogenesis is associated with its periodization. The largest unit of periodization - link. Most animals ontogeny consists of three levels. Each link is divided into several periods. Thus, embryonic link in lower vertebrates divided into two periods - the embryonic and larval, the higher vertebrates three - embryonic, redpaddy and fertilized. There are also transitional States - hatching from the egg shells in reptiles and birds; childbirth in mammals; metamorphosis (see) in many animals.
The initial stages of ontogenesis of the person are characterized by a more rapid pace than in related species (for example, chimpanzees). Next comes the slowdown as antenatal and postnatal development. The slowdown was caused by the intense growth of the brain during prenatal development (the brain of a newborn weighs on average 340 g), and in the first years after birth (the child's brain weighs 2 years over 1 kg). With the growth of the brain associated slowdown bones, partly depending also on the size of the pelvic ring.
As a rule (with the close relationship of the two types of placental) smaller animals have a shorter pregnancy.
Postnatal ontogenesis of man is divided into four periods (age): children, teenagers, youth and adult (Mature).