4 INTR ODUC TION 



thetical character, but a generalized statement of observed fact which 

 may be outlined as follows : — -^ 



In all the higher forms of life, whether plants or animals, the 

 body may be resolved into a vast host of minute structural units 

 known as cells, out of which, directly or indirectly, every part is 

 built (Figs. I, 2). The substance of the skin, of the brain, of the blood, 

 of the bones or muscles or any other tissue, is not homogeneous, as it 

 appears to the unaided eye, but is shown by the microscope to be an 

 aggregate composed of innumerable minute bodies, as if it were a 



Fig. 2. — General view of cells in the growing root-tip of the onion, from a longitudinal section, 

 enlarged 800 diameters. 



a. non-dividing cells, with chromatin-network and deeply stained nucleoli ; b. nuclei preparing 

 for division (spireme-stage) ; t. dividing cells showing mitotic figures; e. pair of daughter-cells 

 shortly after division. 



colony or congeries of organisms more elementary than itself. The 

 name cells given to these bodies by the early botanists, and ulti- 

 mately adopted by nearly all students of microscopical anatomy, 

 was not happily chosen ; for modern studies have shown that although 

 the cell may assume the form of a hollow chamber, as the name 

 indicates, this is not one of its characteristic or even usual features. 

 Essentially the cell is a minute mass of protoplasm, a substance long 

 since identified by Cohn, Leydig, Max Schultze, and De Bary as the 

 essential active basis of the organism, afterward happily characterized 



