The long-standing model of pigment cell origins is implicitly based upon a progressive fate restriction model of neural crest development and postulates the existence of a partially restricted progenitor common to all pigment cell types (i.e., a chromatoblast) [38] .
Sodium is the fourth most abundant element on earth, comprising about 2.6% of the earth's crust; it is the most abundant of the alkali group of metals. It is now obtained commercially by the electrolysis of absolutely dry fused sodium chloride.
In its elemental form, sodium has a silvery-white metallic appearance. It is the sixth most abundant element, making up 2.6 % of the earth's crust. Sodium does not occur in nature as a free element and must be extracted from its compounds (e.g., feldspars, sodalite, and rock salt).