INTRODUCTION 17 



of the chromatin, it may be said that the chromatin is continuous 

 from one cell generation to another. The cells resulting from 

 mitosis may differ greatly in size, but the chromatin seems to 

 be divided equally between them with great exactness. Second, 

 cells are never known to arise except from preexisting cells. These 

 two facts are perhaps the most important for us to keep in mind 

 as we go on to study the more complex problems of fertilization 

 and cell division in the many-celled animals. 



(4) The Cell Theory. --Cells were first described by Hooke, 

 an Englishman, in 1665. The regular arrangement of the com- 



FIG. 5. Cells of cork. Facsimile of a figure by Hooke. (From Farmer 



in Lankester's Zoology.) 



partments in cork (Fig. 5) reminded him of the cells of the monks 

 in a monastery and suggested the term. In 1833 Brown de- 

 scribed the nucleus as a constant cell element, and a few years 

 later Schleiden (1838) and Schwann (1839) advanced the idea 

 that all plants and animals are composed of cells. For many 

 years the cell-wall was considered the important part of the 

 structure, but later the protoplasm within it was recognized as 

 the principal constituent, and the cell was then defined as a 

 mass of protoplasm containing a nucleus (Max Schultze, 1861). 

 The importance attached to the cell theory may be judged 



