Chick - 18 Hours
At 18 hours after fertilization, area pellucida is surrounded by dark area called area opaca.
The primitive streak lies in the middle of the area pellucida by clear line.
At the tail side of the primitive groove is formed in front of the primitive streak end in a small pit. This structure is homologue with dorsal tip.
The formation of notochord starts. The process of gastrulation is about begin