Video - Human development week 3

The third week of human development is marked by the development of the primitive streak, which establishes the first body axis. At around day 14, the process of gastrulation takes place, which involves the epiblast differentiating into the trilaminar embryonic disk consisting of the ectoderm, the mesoderm, and the endoderm.

There is also the formation of the primitive groove or steak groove that runs down the center of the epiblast layer. At around day 17, a group of mesodermal cells forms the notochord, a structure that releases Sonic hedgehog protein that helps to orient tissue differentiation tissue differentiation. At around day 20 to 21, the notochord triggers the process known as neurulation, in which the ectoderm forms the neural plate, later on folding into the neural tube.