Blastula - Single embryo
Blastula is a single layer of the embryo, which is formed by the cleavage of a fertilized egg.At this stage the embryo is composed of blastomeres - the layer of cells surrounding a cavity (blastocoel).The blastula having a cavity inside, consisting of blastomeres of approximately equal size, called tseloblastuloy.It occurs in a wide variety of organisms, coelenterates, sponges, flat and segmented worms, echinoderms, mollusks and chordates.In some species tseloblastuly flagella may form as a result they can get out of the egg shells.
The man blastula is formed in the first week of embryonic development (during crushing zygote).It has a kind of bubble, which distinguish blastocoel (a fluid-filled cavity) and blastomeres surrounding her.Blastomeres can be of two types: small and large dark light.From small light cell
How is the human embryo
In the second week of the embryo cells embryoblast divided into two layers: the outer and the inner.From outer layer formed ektoblastichesky vial, which is filled with amniotic fluid.From the inner layer of cells begins to form endoblastichesky (yolk) vial.The body of the embryo is where these bubbles come into contact.By the end of second week fetus has a length of 1.5 mm.
In the third week of the embryo forms three-layer shield.The cells of the outer layer are shifted to the rear end, forming a so-called primitive streak.It determines the symmetry of the body of the embryo (left and right sides).In the head (front) portion of the primary bundle is formed.As a result of the rapid growth of the primary node and the primitive streak is formed mesoderm - the average germ leaf.At the end of the third week in the outer germ layers appears neural plate - strip of actively growing cells.Later it formed from the sequence of nerve groove, the neural tube from which the developing nervous system.By the end of the third week of the embryo appear axial organs.
the fourth week comes from the isolation of extraembryonic parts formed body segments.In the fifth week, there are the beginnings of arms and legs, in the sixth week, they are divided into the main sections, on the seventh week, there is the emergence of the rudiments of fingers.At the age of 8 weeks the fetus are the main morphological characteristics of man.By the end of the eighth week of tab ends, and ends the embryonic period of development of the organism.After eight weeks, the developing organism is called a fetus.