This was also true for the methods of chemical degradation, carried out in order to find the components of nucleins in their highest state of natural complexity. It was learned for example, that the special kind of carbohydrate present in nucleins was very susceptible to change under the conditions of hydrolysis by acids. Phoebus Aaron Theodor Levine (1869-1940), therefore, used the digestion by a living organism. With E. S. London, he introduced a solution of nucleic acid into, e.g., the gastrointestinal segment of a dog through a gastric fistula and withdrew the product of digestion through an intestinal fistula. Fortunately, the products obtained in such degradations were not new in themselves. The carbohydrate in this nucleic acid proved to be identical with D-ribose, which Emil Fischer had artificially made from arabinose and named ribose to indicate this relationship (1891). The nitrogenous products of the degradation were identical with substances previously prepared in the long study of uric acid. In the course of this study, Emil Fischer established uric acid and a number of its derivatives as having the elementary skeleton of what he called "pure uric acid," abbreviated to purine. Out of Adolf Baeyer's work on barbituric acid came the knowledge of pyrimidine and its derivatives. [Illustration: Figure 15.--ALBRECHT KOSSEL (1853-1927) received the Nobel Prize in Medicine and Physiology in 1910 for his work on nucleic substances, which contain a high proportion of phosphorus. The chemical bonds of this phosphorus in the molecules of nucleic substances were determined in later work. (_Photo courtesy National Library of Medicine, Washington, D.C._)] From these findings, together with what Oswald Schmiedeberg (1838-1921) had established concerning the presence of four phosphate groups in the molecule (1899), Robert Feulgen (1884-1955) constructed the following scheme of a nucleic acid. Feulgen's formula of 1918 is: Phosphoric acid--Carbohydrate--Guanine Phosphoric acid--Carbohydrate--Cytosine Phosphoric acid--Carbohydrate--Thymine Phosphoric acid--Carbohydrate--Adenine Of the four basic components on the right, thymine occurs in the nucleic acid from the thymus gland. Yeast contains uracil instead. The difference between these two bases is one methyl group: thymine is a 5-methyluracil. In all of these basic substances, the structure of urea NH_{2} / C=O \ NH_{2} is involved, and they form pairs of oxidized and reduced states: PURINE PYRIMIDINE (reduced) Adenine + (oxidized) Thymine (oxidized) Guanine + (reduced) Cytosine 3N = CH4 | | 2H--C CH5 || || 1N--CH6 Pyrimidine