THE ANIMAL CELL. 19 



33. Size. While ordinary tissue cells are minute, there is 

 great variation in the size of cells. Many single-celled in- 

 dividuals are visible to the naked eye and egg-cells may be 

 several centimetres in diameter; yet many tissue cells are less 

 than .005 millimetre in diameter. Cells may be very much 

 extended in one or more directions. The outgrowths of nerve 

 cells for example may attain a length of several feet, as when 

 the nerve fibers extend from the trunk to the tips of the toes. 



34. Structure. The following parts are to be distinguished 

 in the typical cell: (i) a general cell substance, partly liv- 



FIG. 3. 



FIG. 4. 



FIG. 3. Diagram showing the principal parts of the cell and something of the 

 protoplasmic architecture as it might appear while living, a, alveoli or spheres in the 

 foam-work (see 17); c, centrosome; cy, cytoplasmic meshwork, containing granules; 

 nu., nucleus; n, nucleolus; v, vacuole; w, cell wall. 



FIG. 4. Diagram showing principal parts of the cell as it appears when killed 

 and stained. The protoplasm shows more of a meshwork (cy), the spaces represent- 

 ing the alveoli, f, formed substance in alveoli. Other letters as in Fig. 3. 



Questions on figures 3 and 4. If these cells are in reality 25 M in 

 diameter, how much are they enlarged in the drawing? (A* is .001 mm.). 

 Identify the various structures referred to in section 34. 



ing protoplasm, partly non-living matter both organic and 

 inorganic; (2) usually a single highly differentiated nucleus 

 which contains living protoplasm and is clearly demarcated 

 from the substance about it; (3) one or more specialized 

 bodies known as centra somes \ (4) a cell wall or membrane 

 (Figs. 3 and 4). 

 The cell-substance or cytoplasm embraces that portion of 



