154 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



hyaline cap.. 



pseuctopodiam 



\ 

 ptemagsl.m o _, 



many types of insect larvae and pupae may also be seen. This brief 

 list includes only a few of the many new acquaintances to be found 

 in a drop of water. 



AmebOj an Animal Cell 



Ameba is the classic representative of a single-celled animal which 

 illustrates the action of living protoplasm. Found in ooze taken 

 from the bottom of small ponds or sluggish streams, it is seen to be 



an irregular and almost 

 transparent cell. When 

 in motion the protoplasm 

 of its body apparently 

 flows out into newly 

 formed bulging projec- 

 tions of the body called 

 pseudopodia (Gk. pseu- 

 dos, false; pous, foot). 

 The cell body consists of 

 two substances, an inner, 

 more fluid, granular por- 

 tion, the endoplasm and 

 a more viscous area, the 

 ectoplasm, on the outside. 

 The whole Ameba is 

 surrounded by a deli- 

 -foodvacuole cate plasma membrane. 

 When the animal moves, 

 the protoplasm appears 

 to flow into the pseudo- 

 podia. According to S. 

 O. Mast of the Johns 

 Hopkins University, 

 when an Ameba is mov- 

 ing in a given direction the endoplasm sol pushes out in a pseudopo- 

 dium and becomes changed to a gel, the "gel" at the other end of the 

 cell becoming a "sol" that moves into the cefl body. This illustrates 

 a characteristic of protoplasm mentioned earlier. 



This cell, like others of its kind, has a nucleus containing chromatin. 

 Certain vacuoles are present, some of which are filled with a watery 

 fluid, others hold food in different states of digestion, while a single 



---■nuclexcs 



li.L.-fooct vacuole. 



Contractile, 

 vacuole— 



Ameba proteus. The direction of progress of 

 the cell is shown by arrows. What happens to 

 the protoplasm in the extreme anterior end during 

 movement. (After Mast.) 



