Development of animals, The formation of gametes gametogenesis in animal, Female and male sex cells gametes in animal
The process of individual development of an organism from birth to death
It consists of two main stages: prenatal (prenatal) and postnatal (postpartum).
Prenatal ontogeny has peredzarodkovoho period (development of gametes and fertilization) embryonic period (embryonic), and fruiting period (fetal)
Postnatal ontogeny consists of juvenile period (youth), mature (reproductive, breeding season), aging and death.
The formation of gametes (gametogenesis)
is the result of meiosis. Meiosis - a peculiar type of cell division in which a decrease in the number of chromosomes. Meiosis consists of two consecutive (meiotic) divisions. In the process of sharing parental chromosomes exchange their identical plots (crossing-over) and are, therefore, signs of both parents. The females by two meiotic divisions (Ovogenesis) produces four daughter cells.
Of these, only one becomes mature germ cells with a haploid set of chromosomes and the other three form napravitelnym cells, which then are reduced. In males, the primary sex cells form four daughter cells that develop into mature sperm (spermatogenesis).
Female and male sex cells (gametes)
are different from each structure. Thus, mammalian egg (ovum) in addition to the nucleus and the cytoplasm has a shell composed of follicular cells, radiant crown and transparent shell. These membranes serve a protective function, provide the necessary metabolism, prevent the penetration of more than one sperm, promote the implementation of the embryo in the uterine wall.
Sperm is the head, neck, body and tail. At the head is the nucleus and the rest of the form machine motion as flagella. Sperm has akrosomnoy device that covers the head, which contains enzymes that dissolve the egg shell, which provides the penetration of sperm into the egg and fertilization. The fertilized egg becomes a zygote that undergoes fragmentation and gastrulation with the formation of germ layers - the source of all structures of the body, which are formed in the process of differentiation.