DEV-4

     By the fifth week of development the lateral wall of the prosencephalon has pushed outward in a blister-like manner, carrying the cental cavity with it to form a new space called the lateral ventricle. The part of the cavity that remains behind, in the mid-line, becomes the third ventricle. There is also a less than obvious subdivision of the space within the rhomencephalon, forming the rostral and caudal parts of the fourth ventricle.
     Thus, we now have 5 cavities within the brain:

1) The lateral ventricle
2) The third ventricle
3) The cerebral aqueduct
4) The rostral fourth ventricle
5) The caudal fourth ventricle

     These spaces will form the ventricular system of the brain, and as we shall see in the next view, the tissue that is the wall of each space becomes a major subdivision of the adult brain.