Human ontogenesis is a process of individual development from the moment of conception to death. The main characteristics are closely interconnected qualitative and quantitative transformations, specific for each stage.
The concept of "ontogenesis" was first introduced by Haeckel (German biologist) in 1866. Different representatives of the animal world are characterized by different development. So, the ontogenesis of insects occurs according to the larval type with subsequent metamorphosis. In animals laying eggs, individual development mainly occurs in eggs laid in the external environment. In placental animals, human development is prenatal. In this case, the process is divided into three periods: postembryonic, embryonic, proembryonic.
The latter is characterized by the formation and development of male and female germ cells - gametes, when merged which forms a fertilized egg.
Human ontogenesis at the embryonic (embryonic) period consists of the stages of the embryo and the fetus.
The post-embryonic period begins after the resolution of labor. This stage lasts a lifetime, ending with death. Human postembryonic ontogenesis is divided into age stages. At each stage, the body undergoes its anatomical and physiological changes. According to experts, the most vulnerable and critical periods are menopause (time of extinction of sexual functions) and puberty (stage of puberty). These are the main features of human ontogenesis.
The early stages of embryonic life are characterized by the emergence of the central nervous system. Ontogenesis of the nervous system in humans continues during the first years after birth.
A plate is formed in the dorsal region of the embryo. Later, a nerve groove is formed, and subsequently a neural tube.
A weekly embryo is characterized by a slight thickening of the tube in the oral region. By the third week, three brain vesicles (primary) are formed in the head region: posterior, middle, and anterior. Of these, the main parts of the brain are formed (diamond-shaped, middle, final). Subsequently, dismemberment occurs in two bubbles (front and back). From the final, hemispheres of the brain and nucleus (subcortical) are formed.
By the third month of embryonic development, the main sections of the central nervous system begin to be determined. These include the cerebral hemispheres, cerebral ventricles, trunk, spinal cord. By the fifth month, the main furrows are distinguished in the cortex (hemispheres). Four weeks later, the predominance (of a functional nature) of the higher departments over the trunk-spinal areas is determined.
The nerve cells of the embryo and newborn are concentrated in the white matter and on the surface of the hemispheres. Due to the increase in surface, cell migration to gray matter begins.
Compared with an adult, in a newborn, the occipital lobe in the cerebral cortex is relatively large. Human ontogenesis in the first five to six years after birth has certain specifics. During this period, the greatest changes occur in the topographic location, shape and number of hemispheric gyrus. By the age of fifteen to sixteen, there is some similarity with adults.
For the postnatal period, changes in the spinal cord are also characteristic. In a newborn, it is longer than in an adult. The spinal cord grows to about twenty years.
In a newborn, the nervous system is not myelinated (sheathed) enough, the arrangement of bundles of nerve fibers is uneven, and they (bundles) themselves are rare.
The functioning of the nervous autonomic system begins in humans from birth. In the postpartum period , fusion in individual nodes and the formation of plexuses in the nervous sympathetic system are noted .