MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedonyuk 2010

BIOLOGY

SECTION 1. BIOLOGICAL BASES OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.3. Biology of Individual Development

Ontogenesis and its Periodization

Ontogenesis (from the Greek ontos - being, genesis - development), or individual development, is The Development of an individual from the moment of Fertilization and The formation of a zygote (in sexual reproduction) until death. Its foundation is the Selection/27.html">Realization of Genetic information at all stages of ontogenesis under specific environmental conditions. Ontogenesis is closely related to phylogenesis (historical development). The regularities of individual development are studied by developmental biology. There are two types of ontogenesis: indirect and direct. Indirect development occurs in a larval form, while direct development occurs in non-larval and intrauterine forms.

Indirect development. Indirect, or larval, development occurs in species whose eggs are poor in yolk. In their development, these organisms have one or more larval stages that structurally differ, to a greater or lesser extent, from the adult form. Larvae lead an active lifestyle and, in most cases, procure their own food, though in some species (mainly parasitic ones) they possess only dispersal Organs (miracidia and cercariae of flukes, coracidia of certain tapeworms). To perform vital Functions, larvae possess a series of provisional (temporary) organs that are absent in the adult state. This type of development is accompanied by transformation - metamorphosis.

Direct development. The non-larval form of direct development occurs in species whose eggs are rich in yolk, which is sufficient to complete ontogenesis (fish, reptiles, birds). The emerging individual has the Structure characteristic of the adult form, but differs in smaller size and immaturity of organs and systems. The intrauterine form of direct development is characteristic of higher mammals and humans. In this type of development, the egg Cell contains almost no nutritive material. All vital Functions of the embryo are carried out through the maternal Organism using a provisional organ, the Placenta. This type of ontogenesis, which is the most phylogenetically recent, ensures the survival of the embryo in the best possible way; however, the newborn organism is not capable of independent life because it requires specific Nutrition from the secretion of the maternal Mammary Glands - milk.

The periods of ontogenesis are embryonic (or prenatal) and postembryonic (or postnatal). In higher animals and humans, division into prenatal or antenatal (before birth) and postnatal (after birth) periods is adopted. In addition to the embryonic and postembryonic periods, a pre-zygotic period is also distinguished in ontogenesis.