40 PRINCIPLES OF PHYSIOLOGY 



stains readily, and the material forming them 

 is termed chromatin or colourable stuff. 

 Further, the minute bodies thus stainable are 

 termed chromosomes, and it is remarkable that 

 the number of chromosomes is almost invaria- 

 bly the same for the cells of each species of 

 animal. The cell also contains matter that 

 is not stainable, or achromatin, as well as 

 matters formed from the substance of the 

 cells, such as droplets of fat, granules of a 

 starchy substance called glycogen, and other 

 bodies, as in secreting cells already mentioned. 

 There is usually a layer of absolutely struc- 

 tureless matter next the cell wall termed 

 hyaloplasm ; and it can be shown that certain 

 matters may pass through this layer while 

 others cannot do so. This is of importance 

 in connection with absorption of matters by 

 the cell and the elimination of matters from it 

 It is important to note that the cell is the 

 theatre of activities, of a physical, chemi- 

 cal, and vital nature, and that probably 

 all the phenomena of life may be manifested 

 by a cell. It often shows irritability, or the 

 power of responding to a stimulus, a property 



