The egg

The ovum (egg) - female sex cell, separated feminine or androgynous ( hermaphroditism) individual and after merging with the male reproductive cell (or at parthenogenesis) giving a new body of the relevant type. The fertilized egg is the zygote is a qualitatively new cell, single-celled embryo of a new subsidiary body. The egg is a large, usually devoid of active mobility cell, it provides a germ of the future of the organism, the bulk of the cytoplasm and especially in oviparous animals, nutrient material (egg yolk). The egg is hundreds of times, and in animals with rich yolk of egg cells is millions of times more sperm. The egg of most animals is surrounded by a protective casing.
Human eggs are developed in the female reproductive gland - the ovary (see). In the process of development of the female reproductive cells oogenesis - there are three phases: 1) the breeding period, during which the primary germ cells, multiply, give rise to a large number of oogonia (predecessors of the egg); 2) the period of growth, when there is an increase in the nucleus and cytoplasm and accumulation yolk inclusions and 3) the period of maturation, including always two cell division, which formed a Mature egg with half the number of chromosomes (reduction of chromosomes) and two or three of the reduction cells.
The foetus of a person (female) number of oogonia as a result of their mitotic division 400 000. By the end of intrauterine development reproduction of oogonia stopped and they turn into primary oocytes. Each oocyte is surrounded by follicular cells of the epithelium, with which it forms a primordial primary follicle. Only a small part of the available in the ovary newborn girl egg Matures and starts the egg. The gap Mature follicle and release of an egg from the ovary that occur from period of pubertyis called ovulation (see the Menstrual cycle). The second division maturation takes place in the oviduct. The movement of the egg on the oviduct in the uterus takes 2-5 days. The egg man has a spherical shape, surrounded by a brilliant shell and layer follicular cells, which form a radiant crown. With a diameter of about 120-150 MK. The egg within 1-2 days retains the ability to fertilize, and then dies off and destroyed. The development of a fertilized egg - see Pregnancy, the Embryo.

The ovum (egg cell) - female sex cell, or the female gamete, separated in sexual reproduction body female (hermaphrodite organisms - bisexual individual) and giving rise after fertilization or without it (at parthenogenesis) the body of the new child generation. Eggs develop in the ovary. By their predecessors during oogenesis are oogonia.
Stopping to reproduce, oogonia turn into oocytes I order, surrounded feeding them follicular cells and enter a period of growth. Two sozrevateley divisions, ripening period), of which one is reduction (see Meiosis), oocyte I order, separating two of the reduction of the bullock, becomes first in the oocyte II order, and then in a Mature egg with haploid (unpaired) chromosome set. The release of an egg from the ovary in female sexual way, is called ovulation (see the Menstrual cycle). In mammals and humans ovulidae still oocyte I order in metaphase first division of the maturation occurring in the initial parts of the oviduct. During fertilization, the sperm penetrates the egg, their haploid set of chromosomes are combined and formed a single-celled embryo - zygote with diploid (double) set of chromosomes. By repeated mitotic division of the zygote gives rise multicellular the fetus.
Unlike sperm (see Sperm),which actively provides meeting with oocyte during fertilization (see) and therefore must; be mobile and free from the supply of nutrients, egg delivers the embryo of a large part of the educational cytoplasm and yolk granules consisting of proteins and lipids (lecithin and other), accumulated growing oocyte in the ovary (phase "large increase"), the egg two poles: the animal, in the area which separates the directing body and where displaced kernel, and vegetative (in his direction shifted yolk). Since the share of yolk higher than the rest of cytoplasm, egg animals, developing in water, are suspended in the last animal pole up. Depending on the quantity and distribution of the yolk inclusions are distinguished: a) solicitudinea cells yolks in them a little, and it is evenly distributed throughout the cytoplasm (many coelenterates, echinoderms, lunatic); such eggs make full uniform grinding; b) letsitele cells, deprived of the yolk (the embryo is fed at the expense of others egg yolk cells, such as flukes); C) telelecture cell - yolk lot and it is concentrated in vegetative parts of eggs (amphibians); such eggs undergo full, but uneven split (animal parts eggs crushed faster and smaller form the cells). Egg named types, making a complete crushing and entirely spent on education of the body of the embryo, called globustelecom. In centralizedly eggs arthropods free from the yolk of the surface layer of cytoplasm of the egg, and he is involved in the crushing of the (partial surface grinding). In many animals (cephalopod mollusks, fish, reptiles, birds, echidna and platypus) egg sharply telelectures type, overloaded yolk that make up most of the mass of eggs; almost free from the yolk and retains the ability to split only a small embryonic disk on the animal pole (partial discipline crushing). Egg performing partial (surface or discipline) crushing, called monoblastny; to build the embryo is part of the egg. Viviparous mammals that occur from ancestors with rich yolk eggs (extinct forms reptiles), in connection with the transition to supply the developing embryo nutrients due to the parent body in the course of evolution of the newly acquired microscopically small (a person 100-150 microns in diameter), poor yolk solicitudinea egg undergoes a complete crushing.
In the cytoplasm of the egg of a number of animals there is a preliminary structure, i.e. areas spent on education of various defined sheets, organs and tissues of embryo. In the processes of fertilization plays the role of the surface of the cortical layer of the cytoplasm of the egg, containing cortical granules; after the sperm penetration last popped to the surface and, merge to form the shell of fertilization, preventing the penetration of the egg other sperm.
There are three kinds of shells of the eggs: primary, produced by the oocyte (shell fertilization), secondary (produced follicular cells) and tertiary (produced by glands in the female reproductive tract, such as protein shell, and the shell of the egg). The egg mammals and humans, surrounded by a transparent membrane (the zona pellucida)produced by the epithelial cells of the follicle, which is developing oocyte in the ovary and, therefore, represent secondary shell. It is permeated with a thin channel, pass through them to the egg cytoplasm cytoplasmic processes of follicular cells. During ovulation, the egg is released from the ovary surrounded by radiant crown (corona radiata) from follicular cells; as progress on the oviduct and especially when meeting with sperm to the egg loses radiant crown (denudation eggs).