Explain the absence of a yolk sac in the development of a human foetus while it is an important structure in the development of birds.

The yolk sac produces red blood cells in the human embryo but its function is quickly taken over by the liver in the foetus. It then degenerates, as it no longer serve any other function.

In the birds, the yolk sacs are very important source nourishment throughout the development of the embryo and cannot be done without. In the human foetus nourishment is provided through the placenta.