32 



COLLEGE ZOOLOGY 



Pseudopodium 



Cell membrane 



Endoplasm 



Ectoplasm 



Figure 11. Structure of Amoeba. Arrows indicate direction of movement. 



and a little of the structure of the animals 

 can be observed. By changing the conditions 

 with respect to temperature, light, etc., one 

 can study their behavior. To obtain a satis- 

 factory idea of the structure of the organ- 

 isms, it is necessary to kill them and treat 

 them vi'ith certain dyes which stain some of 

 the parts, thus making them visible or more 

 distinct than they appear in a living animal. 



Structure (morphology) 



Amoeba proteus (Fig. 11) is only about 

 i/ioo inch (0.25 mm.) in length. It appears 

 under the microscope as an irregular, gray- 

 ish particle of animated jelly that is continu- 

 ally changing its shape by thrusting out and 

 withdrawing little fingerlike processes. Two 

 types of cytoplasm are recognizable in the 

 amoeba, the central part of the body appears 

 to consist of granular protoplasm called 

 endoplasm; surrounding the endoplasm is 

 a thin layer of clear protoplasm called ecto- 

 plasm. Although the ectoplasm is sur- 

 rounded by only a very thin external elastic 

 cell membrane, yet it has been observed 

 that amoebas crawl over each other and 

 never fuse. Within the endoplasm several 



bodies may be seen that are larger than the 

 ordinary granules. One of these, the nucleus, 

 is not easy to see in the living animal, but 

 when stained it appears to be disk-shaped 

 and filled with chromatin granules. The 

 nucleus is thought to play an important part 

 in such fundamental activities of the cell as 

 growth, manufacture and use of foodstuffs, 

 and formation of new cells. If an amoeba is 

 cut into two pieces, the part containing the 

 nucleus may continue to live and reproduce, 

 but the one without the nucleus cannot re- 

 produce itself and soon dies. 



A clear, bubblelike body can often be 

 seen lying near the nucleus; this is known as 

 the contractile vacuole (Fig. 11), because 

 at more or less regular intervals it is carried 

 to the surface, where it contracts and forces 

 its fluid contents out of the body. Other 

 vacuoles may often be seen in the endo- 

 plasm; these may be temporary and may 

 contain food bodies in process of digestion, 

 or they may be more or less permanent. 



When an amoeba is examined with higher 

 magnification, streaming movements may be 

 observed in the endoplasm, indicating that 

 this part of the protoplasm is in a liquid 

 (sol) condition. 



