Ontogenesis is the individual development of an organism from the beginning of its existence to the end of its life.
In the ontogeny of animals, two periods are distinguished – embryonic and postembryonic.
Embryonic (embryonic) development covers the processes from the first division of the zygote to hatching or birth, and in most animals it includes three main stages: cleavage, gastrulation, and organogenesis.
Cleavage is seven to eight consecutive mitotic divisions of the zygote.
During cleavage, daughter cells (blastomeres) do not diverge and do not increase in size. With each subsequent division, their sizes decrease.
Eggs with a small supply of nutrients divide completely, that is, complete cleavage occurs. If the egg contains a large amount of yolk, then partial cleavage is observed – only the cytoplasmic disk divides with the nucleus, and the yolk itself remains unchanged (for example, in birds).
Cleavage ends with the formation of a single-layer multicellular embryo – blastula.
Blastula is a spherical embryo, the wall of which (blastoderm) is formed by one layer of cells, and inside there is a cavity (blastocoel).
After cleavage, the process of gastrulation takes place, which is characterized by the movement of a part of the cellular material from the surface of the blastula inward, to the sites of future organs. As a result of these movements, gastrula is formed.
Gastrula is a two-layered embryo consisting of two germ layers: external (ectoderm) and internal (endoderm).
In the lancelet, gastrula arises by invading the blastoderm into the cavity of the blastocoel.
The internal cavity of the gastrula is called the primary intestine. It is connected with the external environment by an opening (blastopore), which becomes the primary mouth.
At the stage of two germ layers, the development of sponges and coelenterates ends.
In all other animals, development continues, and a third germ layer is formed – the mesoderm. It is formed from the endoderm and is always located between the ecto- and endoderm in the primary body cavity.
Further differentiation of the cells of each germ layer leads to the formation of tissues and organs, i.e., to histo- and organogenesis.
A notochord is formed from the endoderm – an internal skeleton in the form of a flexible cord, located on the dorsal side. Subsequently, the notochord in vertebrates is replaced by the spine, and only in some animals (for example, in cartilaginous fish) its remains are preserved throughout their life.
From the ectoderm, located above the notochord itself, a neural plate stands out. Then the edges of the plate rise and close. A neural tube is formed – the rudiment of the central nervous system. A neurula is being formed.
The neural tube, notochord, and intestines create an axial complex of fetal organs, which determines the bilateral symmetry of the body.
Tissues and organs develop in the same way in all three-layered animals.
From the ectoderm in vertebrates, the nervous system, sensory organs, integumentary epithelium with its glands and derived structures (hair, feathers, hooves, claws, etc.) are formed.
The organs of the digestive and respiratory systems are formed from the endoderm: the epithelium of the midgut, liver and pancreas, gills, lungs, swim bladder, and the thyroid gland.
From the mesoderm, muscle tissue, all types of connective tissue (for example, the dermis of the skin, vertebral bodies), the circulatory system, excretory organs, and gonads are formed.
The embryo develops as a single organism in which all cells, tissues and organs are in close interaction.