he embryo has reached a size of about 8 mm (1/4 inch) in length.
The spinal cord, which until now has been open, is beginning to close.
The first blood cells and blood vessels are developing.
Blood moves through these primitive vessels connected to the yolk sac.
A pipe-shaped heart is formed and begins to beat.
The brain begins to divide into 5 parts.
Optic pits form the start of the developing eye.
The cells destined to be the arms and legs are in place.