STRUCTURE OF A TYPICAL CELL 



21 



parts. If we examine a cell from the older part of a 

 plant we shall find that the wall is thicker, that the cell 

 is larger, and that the protoplasm is confined to a thin 

 layer forming a closed sac in contact with the wall. The 

 middle of the cell is filled by a water vacuole containing 

 a variety of substances in solution. The function of this 

 vacuole is an interesting one. It will be shown later 

 that the materials which are dissolved in the water exert 

 a pressure upon the protoplasmic sac to keep it in close 

 contact with the surrounding cell-wall. This is the chief 

 means of maintaining the rigidity of the softer parts of 

 plants. Animal cells, on the contrary, do not usually 



Fig. 2. — Amoeba, a one-celled animal, n, nucleus; cyt, cytoplasm; 

 /, food vacuole; c.v., contractile or excretory vacuole; g, granular 

 more fluid inner part of cytoplasm; e, outer solider layer of the 

 cytoplasm, the ectoplast; p, pseudopodium or projection of the 

 protoplasm. 



have firm cell-walls, and do not have vacuoles of this 

 type at all. 



A single-celled animal will illustrate other cell-struc- 

 tures. The one called Amoeba, shown in the accompany- 

 ing illustration (Fig 2), is a very simple animal. Its 

 body consists of nucleus and cytoplasm. There is no 

 wall about it, nor does it exhibit any other definite cell 

 organs. Nevertheless, if it is watched, it will be found 

 that it is able to change its outline, that is to say, it 



