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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:
  • 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
  • This is the process by which the neural tube and neural crest are formed
Mesoderm Formation
  • At the end of gastrulation, the middle layer of tissue forming the trilaminar embryonic disc is the mesoderm. It is subdivided into three masses, paraxial mesoderm, located adjacent to the midline and which will differentiate into blocks of tissue called somites; intermediate mesoderm, located lateral to the paraxial mesoderm; and lateral plate mesoderm, which will form the linings of the body cavities and coverings of the visceral organs.
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. During the embryonic period, but in particular the 3rd and 4th weeks, the embryo is at its greatest risk for developmental defects, in particular due to environmental factors. 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.