It was in this treatise also that he made use of the term cell-theory, as follows: "The development of the proposition that there exists one general principle for the formation of all organic productions, and that this principle is the formation of cells, as well as the conclusions which may be drawn from this proposition, may be comprised under the term _cell-theory_, using it in its more extended signification, while, in a more limited sense, by the theory of cells we understand whatever may be inferred from this proposition with respect to the powers from which these phenomena result." One comes from the reading of these two contributions to science with the feeling that it is really Schwann's cell-theory, and that Schleiden helped by lighting the way that his fellow-worker so successfully trod. Modification of the Cell-Theory.--The form in which the cell-theory was given to the world by Schleiden and Schwann was very imperfect, and, as already pointed out, it contained fundamental errors. The founders of the theory attached too much importance to the cell-wall, and they described the cell as a hollow cavity bounded by walls that were formed around a nucleus. They were wrong as to the mode of the development of the cell, and as to its nature. Nevertheless, the great truth that all parts of animals and plants are built of similar units or structures was well substantiated. This remained a permanent part of the theory, but all ideas regarding the nature of the units were profoundly altered. In order to perceive the line along which the chief modifications were made we must take account of another scientific advance of about the same period. This was the discovery of protoplasm, an achievement which takes rank with the advances of greatest importance in biology, and has proved to be one of the great events of the nineteenth century. The Discovery of Protoplasm and its Effect on the Cell-Theory.--In 1835, before the announcement of the cell-theory, living matter had been observed by Dujardin. In lower animal forms he noticed a semifluid, jelly-like substance, which he designated sarcode, and which he described as being endowed with all the qualities of life. The same semifluid substance had previously caught the attention of some observers, but no one had as yet announced it as the actual living part of organisms. Schleiden had seen it and called it gum. Dujardin was far from appreciating the full importance of his discovery, and for a long time his description of sarcode remained separate; but in 1846 Hugo von Mohl, a botanist, observed a similar jelly-like substance in plants, which he called plant _schleim_, and to which he attached the name protoplasma.