Germ

the embryo is a human embryoFetus (synonym embryo) - the body that develops within the egg shells or in the body of the mother. Under embryonic or fetal development in humans refers to the early period of the development of the body (up to 8 weeks)during which from a fertilized egg is formed body with the main morphological features of the man. After 8 weeks, the developing human organism is called a fetus (see).
Embryonic development is divided into a number of periods.
1. The period of single-celled embryo, or zygote, short-term, proceeding from the moment of fertilization before crushing eggs.
2. The period of fragmentation. In this period there is a multiplication of cells, Resulting in the fragmentation of cells called blastomeres. Initially formed a bunch of blastomeres, resembling a berry of a raspberry,- the morula, then single-layer spherical blastula; the blastula wall - blastoderm, cavity - blastocele.
3. Gastrolery. A single germ turns into a double - layer-the gastrula, which consists of external embryonic leaf - the ectoderm and the internal - endoderm. In vertebrates in the course of gastrulation occurs and the third embryonic leaf - mesoderm. During the evolution of the chordates the process of gastrulation complicated by the occurrence of axial complex germs (tab nervous system, axial skeleton and musculature) on the back of an embryo.
4. The period of separation basic rudiments of organs and tissues and their further development. While these processes are enhanced by combining parts into a single developing a whole. From the ectoderm formed the epithelium of the skin, nervous system and partly senses, of endoderm - epithelium of the digestive tract and his cancer; from mesoderm - muscles, the epithelium of the urogenital system and the serous membranes, from the mesenchyme - connecting, cartilage and bone tissue, cardiovascular system and blood.
When conditions change, the development of individual parts of the embryo may change and embryonic sheets can give rise not to those organs and tissues, which would have to evolve from them in normal conditions. Factors that modify the conditions of development, may be the environment (its chemical composition, temperature and other), interaction between parts (cells, the substitutes) of the fetus, and heredity. All these factors are closely linked.diagram early stages of embryo development of the person
Fig. 1. The scheme early stages of embryo development of man and stage of the inner cell mass; b - eight-day embryo; to - day embryo; g - thirteen-fourteen-day embryo. 1 - trophoblast; 2 - blastocele; 3 - cavity amnion; 4 - endoderm cells; 5 - amnion; 6 - the embryo; 7 - yolk SAC; 8 - cell mesoderm; 9 - stalk; 10 - hair chorion; 11-vasarhely General.germ and shell in the early stages of development
Fig. 2. Bud and his shell in the early stages of development (and in successive stages): 1 - the chorion; 2 - allantois; 3 - yolk SAC; 4 - amnion; 5 - vasarhely General; 6 - the umbilical cord; 7 - umbilical vessels; 8 - cavity amnion.


Fig. 3. The germ of a man: and by the end of 4 weeks; - by the end of the 5th week; by the end of the 7th week after fertilization.

The human fertilization occurs in the fallopian tube (in the oviduct). The crushing process should be completed within 3-4 days, when the embryo to go through the fallopian tube to the uterus. The split of the surface of the blastomeres formed shell participating in the power of the embryo,trophoblast. Central blastomeres form the embryoblast, which develops the body of the embryo. During 4-6 days the germ is in the uterine cavity. With the beginning of the second week is an embedding of the embryo into the uterine wall (implantation). In 7.5-day embryo of the ectoderm forms amniotic bubble, part of which facing entoderm, is the ectoderm of the embryo. During this period, the embryo has the form of a shield (disk). At the blastocyst cavity moved cells nazaralieva the mesenchyme and fill it (Fig. 1). Together with trophoblast it forms villous the shell of the embryo is the chorion (see the Placenta). By the end of the second week of entoderm forms yolk bubble. In the result of accumulation mesenchymal amniotic yolk and bubbles are formed amnion and yolk SAC.
"Nazaralieva" part play an important role in the development of the embryo. The yolk SAC of the embryo of the person operates only in the early stages of development, participating in the nutrition of the embryo and performing hematopoietic function. The allantois in oviparous higher vertebrates role urinary bag, the man is a finger-like outgrowth back intestine, which grow blood vessels to the chorion. The amnion - water-shell - forming around the embryo closed bag filled with fluid amniotic fluid. It protects the fetus from harmful influences and creates favorable conditions for its development (Fig. 2).
On the 3rd week of development on the dorsal side of the embryo is allocated dense heavy growing cells, primary strip, head of the Department which thickens and forms the primary (gestosci) bundle. Cell primary strips are immersed in the primary groove, penetrate into the space between the ectoderm and entodermal and give rise to an average of embryonic leaf. On the 3rd week laid spinal chord and neural tube.
On the 4th week, the embryo segregated from nazaralieva parts and as a result of strong growth rolls up into a tube. Simultaneously with this differentiates the mesoderm and formed the segments of the body - somity (Fig. 3, a). In parallel with segmentation are the initial processes of organogenesis (see) and histogenesis. On the 5th week arise rudiments of hands, and then the feet, on the 6th they are divided into main departments, on the 7th buds fingers (Fig. 3, b and 3). At the age of 8 weeks, the embryo becomes the main morphological features of the man in appearance and in the inner organization. Its length from crown to toe) 4 cm, weight 4-5, By the end of the 8th week ending tab bodies of the embryo.