THE GROWTH OF BIOLOGICAL IDEAS 155 



isms. Schleiden described to Schwann the nucleus of plant 

 cells, and Schwann at once recognized it as corresponding to 

 something with which he was familiar in cells of the spinal 

 cord of Vertebrates. The two men repaired forthwith to 

 Schwann's laboratory in the Anatomical Institute of the Uni- 

 versity. Schwann showed his friend the cells of the spinal 

 cord, and Schleiden at once recognized the nuclei as corre- 

 sponding to those with which he was familiar in plants. Due 

 recosrnition must be g^iven to the researches of those who had 

 preceded them in cytological investigations, but this occasion 

 may nevertheless be justly regarded as marking the first gen- 

 eral formulation of the cell theory. The two men published 

 separately. They made big mistakes, but the cell theory— 

 the theory that plants and animals simply consist of cells and 

 the products of cells— must properly be ascribed to them. 



Throughout the forties discoveries followed one another 

 quickly. Mohl came to regard cell division as the usual means 

 of production of new cells. The Swiss zoologist von Kolliker 

 showed that spermatozoa are cells, not mere parasites in 

 semen. His friend and compatriot Karl Nageli witnessed 

 nuclear division and was the first to glimpse the chromo- 

 somes. It was these two friends, more than anyone else, who 

 established one of the profoundest truths in biology: that the 

 egg is itself a cell and gives rise to the cells of the new indi- 

 vidual by repeated division. (It is true that Schwann had 

 already regarded the egg as a cell, but he did not understand 

 how new cells arise.) It was not until the fifties, however, 

 that it became generally accepted that cells never arise except 

 from pre-existing cells, and not until the sixties that proto- 

 plasm was called "the physical basis of life," and the cell "a 

 lump of nucleated protoplasm." 



Much was being learned, then, about the minute structure 

 of animals; something also about the physical properties of 

 protoplasm; and its chemistry was not being neglected. Fried- 

 rich Wohler, the distinguished German chemist, had already 

 synthesized urea from inorganic components in 1828 and thus 

 shown that there was no sharp distinction between organic 



