The third week of development is characterized by several significant events which will set the stage for the subsequent development of organ systems and tissues. These events include:
Gastrulation
  • In this process, the bilaminar embryo becomes a trilaminar disc consisting of three primary germ layers
Formation of the Notochord
  • This establishes a central axis for the embryonic disc and the presence of the notochord is essential for induction of the neural tube
Neurulation
  • This is the process by which the neural tube and neural crest are formed
Somite Formation
  • This is the process whereby axial mesoderm is condensed into blocks of tissue called somites. Somites are the back bone of the development of the vertebral column.
Coelom Formation
  • This is the process of folding of the embryo to produce mesodermally lined cavities with in the embryo.

Many of these processes begin in the third week and continue into the fourth week. By the end of the 4th week, the embryo is poised to begin
organogenesis. Therefore, in the weeks to follow, specific organ systems will be developed. The embryonic period continues until the eighth week of development. From the eighth week until birth is the fetal period, a time characterized by growth of the organ systems laid down in the embryonic period.