Development

While the trophoblast is developing into the placenta, which will nourish the fetus, the inner cell mass is forming the foetus itself. The cells mass differentiates into three layers, each of which will form particular parts of the fetus.

• The ectoderm mainly forms the skin and nervous system
  
• The mesoderm forms bones and muscles and also the heart and blood vessels, including those which are in the placenta.
  
• The endoderm forms mucous membranes and glands.
  

The human development after fertilisation is shown as follows (see figure below).