But there was still another set of studies that led up to this vitamine work. In 1907 E. V. McCollum began the study of nutrition problems at the Wisconsin Experiment Station. At the time he was especially interested in two papers that had been published just previous to his entrance into the problem. One of these papers by Henriques and Hansen told how the authors had attempted to nourish animals whose growth was already complete on a mixture consisting of purified gliadin (the principal protein from the quantity viewpoint in wheat), carbohydrates, fats, and mineral salts. In spite of the fact that the nitrogen of this mixture was sufficient to supply the body needs, as proved by analysis of the excreta, the animals steadily declined in weight from the time they were confined to this diet. The authors had assumed that the gliadin was deficient in a substance necessary to growth (lysine) but since their studies were begun only after the animals had reached maximum growth they expected that the growth factor would not be necessary. Why had their animals declined in weight? The second paper that interested McCollum was by Wilcock and Hopkins. These authors carried out experiments similar to those of the paper just cited but using corn protein (zein) in place of gliadin. This protein had already been shown to be deficient in a chemical constituent known as tryptophan. Animals fed on the zein mixture died in a few days but the inexplicable thing was that when the missing tryptophan was added to the diet the animals lived a little longer but finally declined and died. Why?