Three primitive layers in the ovum can now be differentiated, and from each layer particular parts of the foetus develop.

The ectoderm forms the nervous system, skin and certain lining mucosa.

The mesoderm forms bone, muscle, the circulatory system, and certain internal organs.

Mesoderm forms the mucosa of the alimentary tract, the epithelium of the liver, pancreas, lungs and bladder.

In the inner cell mass two cavities appear:

1. The amniotic sac, which is filled with fluid
2. Immediately below it the yolk sac.