hollow cavity of the embryonic nerve cord is transformed into the narrow central canal of the spinal cord and the ventricles of the brain.Both the central canal and the four ventricles are filled with cerebrospinal fluid.Which is formed by filtration of arterial blood in the central canal and ventricles and then drains into the veins,supplying different parts of the brain with nutrients and hormones and carrying away wastes.In mammals,the cerebrospinal fluid also cushions the brain and spinal cord by circulating between layers of connective tissue that surround the CNS.