52 ORGANISMS OF ONE CELL 



With Amoeba proteus irritability is indicated by more rapid 

 movement, as under the stimulation of increased temperature; 

 or by withdrawing of pseudopodia and rounding out of the body, 

 as under the effects of mechanical, electrical or chemical stimuli. 

 These various responses frequently subserve a useful purpose in 

 capturing food or avoiding difficulties, and represent a proto- 

 type of higher conscious actions. There is absolutely no ground 

 for believing that Amoeba does anything intentionally or wil- 

 fully but all of its activities can be explained on the ground of re- 

 sponses to environmental stimuli. An interesting analogy to 

 vital processes in Amoeba is shown by .a drop of chloroform 

 which, through surface tension, will draw in, and roll up, a 

 filament of shellac. Another species of Amoeba, A. verrucosa, 

 captures and rolls up a filament of Oscillaria in exactly the 

 same way, and the inference is that both processes are due to 

 the same fundamental physical laws. 



Reproduction. In all forms of life the process of reproduction 

 when reduced to its simplest terms is the same, viz. cell division, 

 and the young forms invariably begin life as single cells which, 

 after the stimulus of fertilization or its equivalent, begin to 

 divide. The products of this continued division soon begin to 

 differentiate into tissues and organs, the various phenomena 

 constituting the subject matter of the science Embryology. 

 In Amoeba proteus, however, the cell is never more than a single 

 unit and might at all times be considered the equivalent of an 

 egg. There is evidence, although proof is not certain, that only 

 at definite periods is Amoeba really similar to an egg cell, and 

 requiring fertilization for its continued activities. At other 

 times the cell divides as does the fertilized egg of other animals, 

 but, unlike the products of cleavage of the egg, the daughter cells 

 of Amoeba do not remain attached to one another but separate 

 and live as independent organisms similar to the parent. Here 

 then, as with the yeast cell, reproduction is reduced to its lowest 

 terms, simple division. In this process the nucleus of the cell 

 first divides and then the cell body (Figs. 19 and 20). 



Encystment. When the environmental conditions become 

 unsuitable for life, an Amoeba will secrete about itself a wall or 



