This phosphatide is by far the best-known lipoid. It occurs in the brain, the heart, the liver, and in the yolk of the eggs of many animals; and either lecithin or a substance so nearly like it in character as to be regarded by most investigators as identical with it, is present in small, but constant, quantities in nearly all seeds, especially those of leguminous plants. In many legume seeds, it constitutes from 50 to 60 per cent of the "ether extract," or "crude fat," which can be extracted from the crushed seeds, using ether as the solvent. Lecithin is a glyceride. Only two of the (OH) groups of the glycerol are replaced by fatty acids, however; the third being replaced by phosphoric acid, H_{3}PO_{4}, or PO(OH)_{3}, which, in turn, has one of its hydrogen atoms replaced by the base _choline_. Choline is a nitrogenous base, or amine, which may be regarded as ammonium hydroxide with three of its hydrogen atoms replaced by methyl groups and the fourth by the ethoxyl group, the latter being the ethyl group with an OH in place of one of its hydrogens. Thus, Ammonium hydride Choline H H CH_{3} C_{2}H_{4}OH \ / \ / H-N CH_{3}-N / \ / \ H OH CH_{3} OH Without the choline, lecithin would be a di-fatty acid derivative of glycero-phosphoric acid. These relations may be seen in the following formulas: Glycerol Glycero-phosphoric acid Fatty Acid CH_{2}OH CH_{2}OH OH HOOC·R | | / | | / CHOH CH--O--P=O | | \ | | \ CH_{2}OH CH_{2}OH OH Choline Lecithin HOC_{2}H_{4} CH_{2}OOC·R OH \ | / \ | / (CH_{3})_{3}[trb]N CH---O---P=O C_{2}H_{4} / | \ / \ / | \ / N[trb](CH_{3})_{3} HO CH_{2}OOC·R O / HO fatty acid / Or, glycerol-fatty acid + Choline = Lecithin + H_{2}O \ phosphoric acid There are many different possible linkages of the constituent groups which make up the lecithin molecule. In the first place, if the (OH) groups of the glycerol molecule be numbered (1) and (2), thus, CH_{2}OH (1) | CHOH (2) | CH_{2}OH (1)