Somite 1 now has an anterior boundary. 4-somite stage, dorsal view, focus is on the notochord at the level of the boundary between somites 2 and 3. Note at the top how the brain rudiment and underlying axial mesoderm prominently indent the yolk cell in the midline. The midbrain rudiment lies just dorsal and posterior to optic primordium. The segmental plate, developing paraxial mesoderm posterior to the somite row, is clearly delineated.
Somites begin to take on a chevron shape.
The yolk cell begins to look like a kidney-bean, heralding formation of the yolk extension. Note at the top the shape of the brain primordium, at the level of the midbrain. The arrow shows Kupffer's vesicle. Kupffer's vesicle is also nearly in focus. The otic placode begins to hollow.