CHAPTER VIII. 



CELLS AND TISSUES. 



The present chapter, which may better be omitted on first 

 reading, is intended to give a more detailed account of cells 

 and their association into tissues which have been mentioned 

 frequently in Chapters IV and VII. For this chapter a micro 

 scope should be used. 



The term cell suggests honeycomb ; in fact the cells first 

 described were regarded as small cavities surrounded by walls, 

 but the more essential living part of the cell is that which the 

 wall contains. 



In atypical cell there may be found (i) the protoplasm, 

 (2) the cell sap, (3) the cell wall. 



(i) Protoplasm mentioned in Chapter VII as the living 

 part of the cell, includes : — 



(i) The cytoplasm, a gelatinous substance having the ap- 

 pearance of the white of an egg. It is a complex structure and 

 consists of proteids — that is of organic compounds made up 

 of carbon, hydrogen, oxygen, nitrogen, sulphur, and usually 

 phosphorus. 1 When living it is in constant activity, undergoing 

 continual breaking down and renewal and is besides in cease- 

 less motion. The rate of movement depends to a large extent 

 upon temperature. 



Young hairs of Indian corn leaves reveal this motion if 

 examined under a microscope. The protoplasm here circu- 

 lates to and fro from the wall to a stationary body, called 



(ii) The nucleus. This is a body of the utmost importance. 

 It is complex in composition, complicated in structure, and is 



^ Li\'ing and dead protoplasm differ in chemical composition. 

 63 



