Question

Describe the development of man upto the formation of three germ layers.

Answer 1

• Gastrulation is the formation of the three primary germ layers occurs during the first two weeks of development. Gastrulation takes place after cleavage and the formation of the blastula and the primitive streak. It is followed by organogenesis when individual organs develop within the newly-formed germ layers.
  
• The primitive streak, a linear band of cells formed by the migrating epiblast, appears, and this marks the beginning of gastrulation. Then the primitive streak which establishes bilateral symmetry. A primitive node (or primitive knot) forms in front of the primitive streak which is the organizer of neurulation. A primitive pit forms as a depression in the centre of the primitive node which connects to the notochord which lies directly underneath. The node 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 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. This ingression sees the 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 pushed out of the way and goes on to form the amnion. The epiblast keeps moving and forms a second layer, the mesoderm. The epiblast has now 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 are the ectoderm, mesoderm, and endoderm, and are formed as three overlapping flat discs.