OUTSTANDING BIOLOGICAL ADVANCES 21 



From this point ideas began to broaden and other 

 new relations were discovered. The minute struc- 

 ture of the cell was investigated, the centrosome and 

 the chromosomes were discovered, the behavior of 

 these parts and of the nucleus were observed. The 

 phenomena of fertilization of the egg, the mechanism 

 of heredity and the seat of hereditary qualities were 

 studied as problems pertaining to cell life. 



So many sweeping conclusions resulted that a 

 new branch of biology having to do with the phenom- 

 ena of cellular life was organized. This division was 

 called cytology and the late Theodor Boveri (1866- 

 1916) (Fig. 4), was perhaps its leading representative 

 on the zoological side. 



In its modern statement the cell-theory has come 

 to embrace four aspects which successively have been 

 developed. These are : the conceptions of the cell as a 

 unit of structure; the cell as a unit of physiology; the 

 cell in development and in heredity. The title of 

 Wilson's, now classic, book on The Cell in Develop- 

 ment and Inheritance carries this idea. (For com- 

 ments on the cell in inheritance see p. 43.) 



From these considerations it becomes evident 

 that the cell-theory and the protoplasm idea are 

 broad generalizations that influenced the entire 



