Development of the nervous system in humans optical vesicle (which will eventually become the optic nerve, retina and iris) forms at the basal plate of the prosencephalon. In the fifth week, the alar plate of the prosencephalon expands to form the cerebral hemispheres (the telencephalon). The basal plate becomes the diencephalon. The diencephalon, mesencephalon and rhombencephalon constitute the brain stem of the embryo. It continues to flex at the mesencephalon. The rhombencephalon folds posteriorly, which causes its alar plate to flare and form the fourth ventricle of the brain. The pons and the cerebellum form in the upper part of the rhombencephalon, whilst the medulla oblongata forms
Medulla (hair) Medulla (hair) The medulla is the innermost layer of the hair shaft. This nearly invisible layer is the most soft and fragile, and serves as the pith or marrow of the hair. Scientists are still uncertain about the exact role of the medulla, but they speculate that it is primarily an extension that is more prominent in depigmented (grey or white) hair Human hair is unique in that its medullary index (the fraction of the hair shaft's diameter that the medulla occupies) is very small: generally less than 1/3. Other species will have much larger ratios, usually at least 1/2.