SOME MODERN VIEWS OF THE CELL. 605 



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 re- 

 sults, 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 investiga- 

 tions of animal tissues, in which he happily recognized the funda- 

 mental 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 com- 

 pletely 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, recog- 

 nized its presence in all living plant cells, while the latter bot- 

 anist, two years later, first called it jjrotoplasm, 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 con- 

 tractility and other supposedly characteristic properties of sar- 

 code, he felt justified in saying with much certainty that "the 

 protoplasm of the botanists and the contractile substance of the 

 zoologists, 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 de- 

 served. 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 



