20 



ZOOLOGY 



34. 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 



Fig. 4. 



Fig. 



5- 



nu.- 



cy--.- 



Fig. 4. 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 §18); c, 

 centrosome; cy, cytoplasmic meshwork, containing granules; km., nucleus; n, nucleolus; u, vacuole; 

 If, cell wall. 



Fig. 5. Diagram showing principal parts of the cell as it appears when killed and stained. The 

 protoplasm shows more of a meshwork {cy), the spaces representing the alveoli. /, formed sub- 

 stances in alveoli. Other letters as in Fig. 4. 



Questions on figures 4 and 5. — If these cells are in reality 25 /x in diameter, 

 how much are they enlarged in the drawing? {ji is .001 mm.). Identify the various 

 structures referred to in section 35. 



extended in one or more directions without possessing great 

 bulk. The outgrowths of nerve cells for example may attain a 

 length of several feet, as when the nerve fibres extend from the 

 trunk to the tips of the toes. 



35. Structure. — The following parts are to be distinguished 

 in the typical cell: — (i) a general cell substance, partly living 

 protoplasm, partly non-active matter either organic or in- 

 organic; (2) usually a single highly differentiated nucleus 

 which contains living protoplasm and is clearly demarcated 

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

 bodies known as centrosomes; (4) a cell wall or membrane 

 (Figs. 4 and 5). 



The cell-substance or cytoplasm embraces that portion of 

 the protoplasm outside the nucleus. This is apparently made 



