Delamination of endoderm precursor cells
Primitive streak formation
Primitive groove formation
Different parts of the embryo are at different stages of early development at the same time
Differentiation of the Neural Tissues
Random cells in the disc delaminate, and ďfallĒ into the space between the disc of cells and the yolk beneath. These cells then go through convergent extension, and become the endoderm.
The first indication of embryonic development is the formation of the primative streak, a shallow groove that begins at one end of the embryonic mass and extends across the disc shaped mass.
This primative streak develops into the primative groove, through which cells begin to ingress. An integral aspect of this stage is the Hensenís Node, which begins at the most posterior end of the cell, and as the groove progresses, so does the Node.
Once Hensenís Node reached the anterior-most end of the embryo, it again recedes, leaving behindthe head process-- cells necessary for development of the head region.
Once Hensenís Node begins to recede, the cells it leaves behind begin to differentiate to form structures. Therefore, as Hensenís Node recedes, cells at one end of the embryo have already begun to form structures, while cells at the end through which Hensenís Node passed most recently are still just beginning to differentiate.
Finally, the neural tissues are differentiated, and form the notochord and neural tube.