He considered this a case of a real synthesis of proteins from the products of its hydrolytic cleavage. This interpretation was not generally accepted and received a different interpretation by Bayliss and other workers. Gay and Robertson were able to show that paranuclein when injected into an animal will sensitize guinea-pigs for anaphylactic intoxication for either paranuclein or casein and apparently indiscriminately. The products of complete peptic digestion of casein had no such effect, but the synthetic product of this digestion obtained by Robertson’s method has the same specific antigenic properties as paranuclein, thus making it appear that Robertson had indeed succeeded in causing a synthesis of paranuclein with the aid of pepsin from the products of digestion of casein by pepsin. There are a few statements in the literature to the effect that the specificity of organisms might be due to other substances than proteins. Thus Bang and Forssmann claimed that the substances (antigens) responsible for the production of hemolysis were of a lipoid nature, but their statements have not been confirmed, and Fitzgerald and Leathes[52] reached the conclusion that lipoids are non-antigenic. Ford claims to have obtained proof that a glucoside contained in the poisonous mushroom _Amanita phalloides_ can act as an antigen. But aside from this one fact we know that proteins and only proteins can act as antigens and are therefore the bearers of the specificity of living organisms. [52] Fitzgerald, J. G., and Leathes, J. B., _Univ. Cal. Pub._, 1912, “Pathology,” ii., 39. Bradley and Sansum[53] found that guinea-pigs sensitized to beef or dog hemoglobin fail to react or react but slightly to hemoglobin of other origin. The hemoglobins tried were dog, beef, cat, rabbit, rat, turtle, pig, horse, calf, goat, sheep, pigeon, chicken, and man. [53] Bradley, H. C., and Sansum, W. D., _Jour. Biol. Chem._, 1914, xviii., 497. 6. It would be of the greatest importance to show directly that the homologous proteins of different species are different. This has been done for hemoglobins of the blood by Reichert and Brown,[54] who have shown by crystallographic measurements that the hemoglobins of any species are definite substances for that species. The crystals obtained from different species of a genus are characteristic of that species, but differ from those of other species of the genus in angles or axial ratio, in optical characters, and especially in those characters comprised under the general term of crystal habit, so that one species can usually be distinguished from another by its hemoglobin crystals. But these differences are not such as to preclude the crystals from all species of a genus being placed in an isomorphous series (p. 327).