8 INTRODUCTION: THE FIELD AND PROBLEMS OF BIOLOGY 



tance of cells and at once attracted the attention of many other micro- 

 scopists. New observations were made, many more materials were 

 examined, and the structure of the cell itself was scrutinized. It was dis- 

 covered that some tissues — bone and cartilage, for instance — are built 

 not of cells but very largely of substances produced by bone-forming and 

 cartilage-forming cells. The cell was found not always to have a wall but 

 to consist more essentially of a peculiar stuff called protoplasm. It was 

 also found that cells do not crystallize out of noncellular stuff but instead 

 are invariably formed by the division of a previously existing cell. 



4. Establishment of the Corrected Hypothesis. By about 1860, as the 

 result of the observations and experiments of many talented workers, the 

 cell theory had been restated somewhat as follows: All living things are 

 composed of cells and cell products; the cell consists of a bit of protoplasm 

 containing a nucleus ; and cells are always derived from other previously 

 existing cells. This statement still stands today and is now accepted as 

 entirely substantiated. We no longer speak of the cell hypothesis, but 

 of the cell doctrine or cell principle. 



