Originally Posted by Latenter

Gestational age: 4 weeks old. 29-35 days from last menstruation.
Embryonic age: Week nr 3. 2 weeks old. 15-21 days from fertilization.

* A notochord forms in the center of the embryonic disk. (day 16 of fert.)
* Gastrulation commences. (day 16 of fert.)
* A neural groove (future spinal cord) forms over the notochord with a brain bulge at one end. Neuromeres appear. (day 18 of fert.)
* Somites, the divisions of the future vertebra, form. (day 20 of fert.)
* Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fert.)