CHAPTER II. 



GENERAL VIEW OF THE STRUCTURAL CFIARACTERS OF 

 ANIMAL ORGANISMS. 



THE parts played in Physiology by cells are so many and so 

 important that it is necessary at the very outset to consider their 

 properties somewhat in detail. 



FIG. 1. 



Cells from the root of a plant (X550). 1. Showing youngest cells with 

 thin walls (w), filled with protoplasm and containing nucleus (N) and nu- 

 cleolus (N / ). 2. Older cells with thicker walls with vacuoles and cell-sap 

 (s). 3. Shows further diminution of protoplasm and increase in cavity (s) 

 in proportion to the growth of the cell wall (w). 



The demonstration of the cellular structure of plants was first 

 made in 1832 by a distinguished German botanist named Schlie- 



