5th Weeks
- 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.)