Four weeks after conception, the tiny being which is on its way to becoming a person measures about 6cm in length. This is about a quarter of an inch. The four week embryo has no arms or legs. The region of its face is strange and the entire shape of the organism resembles a new moon more than a person. A bulge produced by the beginning brain and another by the heart are the most conspicuous features at this stage.

The brain is a hollow organ. The future eyes appear as thickened disks of tissues associated with the forward portion of the brain, one on each side.

The digestive organs consist of a simple tube which runs the full length of the embryo. The lungs are beginning to develop as buds of tissue near the heart, and the liver is forming as a branch of the intestinal tube and just below the stomach.