MODERN BIOLOGY 395 



between animal and plant, which the biology of preceding ages had realized 

 in the fact that animals possess a vascular system, which plants lack; the 

 plants' "gefassloses Wachstum' was accounted for by the cell-structure or, 

 as it was then called, the plant's composition of independent units. Now, 

 Schwann had discovered in the notochord of tadpoles cells provided with 

 nuclei, similar to the plant-cells, and both there and in the embryonic 

 cartilage he believed he saw a process of cell-reproduction such as Schlciden 

 had described. This induced him to look for cells in all the tissues of the 

 animal body, and by examining these in the embryonic stage and afterwards 

 following their development he succeeded in establishing the fact of cell- 

 structure even in tissues that in a state of full growth show little or no trace 

 of any such structure. It is not difficult to realize the great influence that 

 this discovery was to have on the tissue theory, and what follows will make 

 it still clearer. Of still greater significance for the future, however, was his 

 general cell-theory, according to which, as he says, "one common principle 

 of evolution is laid down for the most highly differentiated elementary parts 

 of the organisms, and this principle of evolution is the cell-formation." 

 This conception of the cell as a general unit of life and as a common basis 

 for the vital phenomena in both the animal and the vegetable kingdom was 

 immediately and universally accepted; so self-evident did its truth seem to 

 be that it met with hardly any opposition, and in fact became the foundation 

 on which since then both animal and vegetable biology have developed. It 

 is thanks to this theory that the present age has been able to work out its 

 conception of life-phenomena as a connected whole; without Schwann, 

 Darwinism would hardly have been victorious. 



In its details, however, Schwann's cell theory is very primitive; he not 

 only embraces Schleiden's belief in a free cell-formation out of moisture, but 

 takes it further. Out of moisture is concentrated, first the nucleolus, then 

 the nucleus, and finally the cell; this process is explicitly compared with 

 crystallization, and the whole concludes with reflections as to whether the 

 hollow form of the cell might not be accounted for by the "Imbibitions- 

 fdhigkeit" of its component parts — in modern terminology, its colloidal 

 qualities; according to him, then, the cell-formation would be a kind of 

 crystallization in non-crystalline elements. For the essential part of the cell 

 is, in Schwann's view, its hollowness; in its essence it is a space surrounded 

 by walls; its content is a moisture, which runs out if the wall is damaged, 

 while the nucleus is a transitory formation, which disappears in later stages 

 of development. These views were in their essentials corrected in the im- 

 mediately succeeding future. For the rest, Schwann made his cell-formation 

 theory the basis of a general theory of life, which proved to be considerably 

 more materialistic than that of his master, J. Miiller; as a devout Christian 

 he believed in the world's serving a purpose given it by the Creator, but in 



