The early folding of the cranial end now appears as a more complex series of folded regions, though the Neural layers of the CNS have only just begun to form. Neural crest derivatives are now spread throughout the embryo as ganglia and other cell types. Cranial ganglia are much larger and have connections both centrally and projecting to the periphery. High power images of this stage in the next image set show more detail of the developing brain and spinal cord. 4th ventricle- ventricular cavity within the rhombencephalon .
Space is continuous with ventricular system of the brain.
Connects centrally with the dorsal horn of the spinal cord. Generated from neuroepithelial stem cells in ventricular zone of neural tube. Form astrocytes, oligodendrocytes. In the brain it is the outer layer, in the spinal cord it is inner layer. Expression of different Hox genes along neural tube defines rostral-caudal axis and segmental levels.