II. AMOEBA 



Amoeba is a particularly interesting and important type for 

 biological study because it is one of the most primitive and simple 

 living organisms known. It is a unicellular animal, and its struc- 

 ture is such that, with the aid of the microscope, it is possible to 

 observe and study a tiny bit of that unique living substance, proto- 

 plasm, stripped of all the complicating structural features which 

 tend to obscure it in the higher forms of life. Although practically 

 devoid of structural differentiation, an Amoeba is able to carry on 

 all the essential life processes with great success and efficiency. In 

 the final analysis, therefore, this tiny animal cannot be regarded 

 as being essentially simple in its ultimate structure. This is very 

 apparent when we consider that the minute drop of protoplasm, 

 which constitutes the Amoeba, is not only the vehicle but also 

 the primary directing force for an amazing array of complicated 

 processes which are necessary for life. 



A. Structure of an Amoeba 



Under the high power of the microscope a living Amoeba is 

 revealed as a tiny, irregular drop of a rather fluid material which 

 is almost transparent. Continued observation soon shows that a 

 streaming movement is continually occurring, and this results in 

 a rapid change in the shape of the animal. In its very simple 

 form, the Amoeba lacks one of the parts which, as has been noted 

 in the previous chapter, is a usual feature of cell structure, 

 namely, a definite cell wall. The protoplasm, however, is differ- 

 entiated into the cytoplasm and nucleus which constitute the 

 essential parts of cellular structure in all organisms. Further- 

 more, the cytoplasm of an Amoeba consists of a thin outer layer 

 of ectoplasm, which serves as a simple type of protective layer, 

 and an inner region of endoplasm which constitutes the bulk of 

 the animal. 



The ectoplasm shows a clear, homogeneous structure and, in 

 an active specimen, may be described as a rather viscous, trans- 



19 



