158 



THE LOWEST ORGANISMS AND THE CELL 



It is much easier to understand the structure of the plant 

 cell by comparing it with one of the simplest of the one-celled 

 animals ; so at this point there will be given a brief account of 

 one of the best-known protozoans, the Amoeba. 



194. The Amoeba.* The Amoeba under the microscope appears 

 as a minute, irregularly shaped body of a jelly-like consistency. 

 Its form when active constantly changes. A finger-like extension 



or process is thrust 

 out from one side 

 (Fig. 167,^) and the 

 somewhat granular 

 protoplasm flows into 

 this from neighbor- 

 ing regions. Other 

 processes are succes- 

 sively withdrawn, so 

 that the protoplasm 

 actually moves or 

 flows slowly forward 

 into the newly formed 

 lobe, and thus the 

 Amceba glides along. 

 There is present in the 

 protoplasm a denser 

 protoplasmic struc- 

 ture termed the nucleus, which is known to be the center of 

 very important activities in the cell. The protoplasm also con- 

 tains numerous small granules, and frequently large food parti- 

 cles, and there are also globules, called vacuoles, of a watery 

 fluid, which appear and disappear in the thicker substance. 

 Such is the structure of a typical cell, which may be defined as 

 a small mass of protoplasm containing a nucleus. 



* To THE INSTRUCTOR : If material of Amceba is available, its study will 

 furnish an excellent introduction or accompaniment to laboratory work on 

 the plant cell. 



FIG. 167. The Amceba 



A, an individual moving in the direction of the 

 arrows; n, nucleus ; v, pulsating vacuole; /, food 

 body. B, the process of cell division by constric- 

 tion, a nucleus in each half. H, after Jordan, 

 Kellogg, and Heath 



