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. 



niu 



cy...- 



= a 



FIG. 4. Diagram showing the principal parts of the cell and something of the protoplasmic 

 architecture as it might appear while living. , alveoli or spheres in the foam-work (see 18); c, 

 centrosome; cy, cytoplasmic mesh work, containing granules; nu., nucleus; n, nucleolus; v, vacuole; 

 w, cell wall. 



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

 protoplasm shows more of a mesh work (cy), the spaces representing the alveoli. /, formed sub- 

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



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

 how much are they enlarged in the drawing? (jj. 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 severaj, 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 both organic and 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 



