Yet the fact that his theories turned attention to the cell contents also, and the fact that the most conspicuous object in almost any animal cell is its nucleus, made these most fruitful in their results, as other mistaken theories have often been. The analogy between plant and animal cells, suggested by the presence of nuclei in both, led Schwann to thorough and profound investigations of animal tissues, in which he happily recognized the fundamental importance of the study of the development of a tissue for the elucidation of its nature. In this way he showed that animal tissues are made up of elementary units comparable with the cells of plants. The final recognition by Schleiden and Schwann, at the end of the fourth decade of the century, that all organisms consist wholly of cells and the products of their activity laid the solid foundations of the cell theory and brought animals and plants into new and most suggestive relations.
Studies of the nucleus had necessarily drawn attention to the granular substance which surrounds it and more or less completely fills the cavity of the cell, and it had already been called "plant mucus" by Schleiden. In 1844 Naegeli determined it to be a nitrogenous substance, and, together with von Mohl, recognized its presence in all living plant cells, while the latter botanist, two years later, first called it protoplasm, or primitive substance. Up to this time the leading naturalists believed in impassable barriers and inherent differences between plants and animals. One of these supposed distinctions was the rigid and immotile character of plants as compared with the motile and contractile power of animals. The special contractile substance of animals, which was recognized as homogeneous or finely granular and albuminous, had already been called "sarcode" by Dujardin. But when, in 1850, Cohn showed that some plant cells possess no membrane and that their protoplasm shows the contractility and other supposedly characteristic properties of sarcode, he felt justified in saying with much certainty that "the protoplasm of the botanists and the contractile substance of the zoöogists, if not identical, must yet be structures in a high degree analogous." But this conclusion, expressed incidentally in a paper on another subject, did not receive the attention it deserved. It was not until thirteen years later, when the way had been prepared by the work of De Bary on the Slime Molds and of Haeckel on the Radiolarians, that biologists were convinced by Max Schulze's masterly discussion of the subject of the identity of the substances in question. With the new view of the identity of all living substance went a radical change in the conception of the cell. The cell membrane, which is rarely present in animals, was relegated to the list of unessential constituents, and the vital center was transferred in mind to the cell contents, where it has
