The number of positive facts which Müller brought to light in the most diverse departments of science is almost beyond calculation, and yet it is astonishing how seldom the slightest error, or even an incomplete observation, can be found in his work. On the other hand, it has happened, over and over again, that when the correctness of his observations in the beginning seemed according to other investigators to be dubious, they have come eventually to be acknowledged as representing the truth. As a rule, he went over every set of observations three times. During the second series he wrote about them. He always repeated the experiments on which his observations were founded while his material was going through the press. His manuscripts were a mass of corrections; notwithstanding this, his proof sheets were the despair of the printers. Müller accomplished all this only by the most careful husbanding of his time. He knew how to make use even of the ends of hours and brief intervals which others waste without a thought about them. He used to call these periods of short duration between the duties "the gold-dust of time," and said that he did not wish to lose a particle of it. In the quarter of an hour between two lectures it was not an unusual thing to find that he took up some dissection at which he was engaged, or continued his work sketching the observations that he had been making during the previous day. How thorough was Müller's work in everything that he devoted himself to can be gathered from certain excursions into pathology, which was, after all, only a side issue in his work, and to which he gave very little serious attention. Müller's assistant in the Museum of Berlin, and one of his {241} favorite pupils, Schwann, made a series of what Virchow calls comprehensive and magnificent investigations on the cell structures of the animal tissues, on which progress in pathology so essentially depends. Müller followed up these discoveries, and, to quote Virchow once more, he was in this matter the authority of authorities; for the medical world owes to him practically all its knowledge of tumors. Müller first demonstrated the harmony which existed between the pathological and the embryonic development of tumors.