Question

Describe the development of man up to the formation of three germ layers.

Answer 1

Hint: The germ layer, of the three primary cell layers, formed in the earliest stages of embryonic development, consisting of the endoderm (inner layer), the ectoderm(outer layer), and the mesoderm (middle layer). The germ layers always form during the process of gastrulation, when the hollow ball of cells that constitutes the blastula begins to differentiate into more-specialized cells that become layered across the developing embryo.

Complete answer:
The process of formation of 3 primary germ layers during the first 2 weeks of development is called gastrulation.
It takes place after the cleavage and formation of blastula. It is followed by organogenesis where the organs develop within the newly formed germ layers.
The linear band of cells that is formed by the migrating epiblast, appears, and begins the gastrulation process. Also a primitive node or a knot is formed in front of the primitive streak which is the organizer of neurulation.
The primitive pit is formed as a depression in the center of the primitive node which connects to the notochord which lies directly underneath. The node which is present has arisen from epiblasts of the amniotic cavity floor, and it is this node that induces the formation of the neural plate which serves as the basis for the nervous system.
The epiblast in that particular region moves down into the streak at the location of the primitive pit where the process called ingression, which leads to the formation of the mesoderm takes place. The ingression makes sure cells from the epiblast move into the primitive streak in an epithelial-mesenchymal transition; epithelial cells become mesenchymal stem cells, multipotent stromal cells that can differentiate into various cell types.

The hypoblast is moved out of the way and goes on to form the amnion. The epiblast moves and forms a second layer, the mesoderm. The epiblast is currently differentiated into the three germ layers of the embryo so that the bilaminar disc is now a trilaminar disc, the gastrula.
The three germ layers that found are the ectoderm, mesoderm, and endoderm, and are formed as three overlapping flat discs.

Note: The endoderm is the germ layers formed during animal embryogenesis. Cells that are migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm. It is the first layer to be developed.