The center of the neural tube becomes whereas the outside tissue becomes

During the embryonic development, the center of the neural tube becomes ventricles of the brain, and the outside tissues becomes the brain tissues.  

The ventricles of the brain are the communication network composed of cerebrospinal fluid filled cavities located in the parenchyma of the brain. It is composed of 2 lateral ventricles, third and fourth ventricle.

b brown mice / white sand dunes