I AMOEBA 9 



about by a parent individual simply resolving itself into two daughter 

 individuals. This simple process of fission is not the only method of 

 reproduction occurring in Amoeba proteus. During the winter season 

 when conditions are unfavourable to the continued activity of the Amoebae 

 they hibernate in the seclusion of a protective cyst which they form round 

 themselves and during this period of seclusion they reproduce by a method 

 other than that of simple fission. 



At the commencement of this period of encystment the Amoeba is 

 seen to become sluggish in its movements and there accumulates 

 over its surface a thick mass of sticky slime mixed with particles of 

 debris — derived partly from the mud outside, partly from solid faecal 

 matter which the Amoeba extrudes completely at this time. The Amoeba 

 assumes a spherical shape and now secretes all over its surface a tough 

 membrane, usually in two distinct layers with a space between. The 

 three layers indicated — the outer slimy and the two inner membranous 

 layers — constitute the protective cyst in which the Amoeba hibernates. 



During the period of encystment the nucleus of the Amoeba under- 

 goes mitosis repeatedly until there is a large number of nuclei — commonly 

 from 75 to loo — and the cytoplasm now breaks up into as many pieces, 

 each containing a nucleus. When conditions become favourable, in 

 the early spring, these issue from the cyst as so many small Amoebae, 

 which proceed to lead their normal life and gradually grow to the full size. 



What has been said up till now regarding Amoeba has been ascertained, 

 by the method of simple observation, but it is possible to add considerably 

 to the knowledge so obtained by making use of the method of experiment, 

 i.e. by introducing some sudden change into the conditions under which 

 the Amoeba is living and ascertaining how it reacts to the particular 

 change of conditions. Various kinds of change may be made use of to 

 serve as a stimulus to the Amoeba — mechanical, chemical, thermal, 

 electrical — and so on. Of such it may be said in general that the Amoeba 

 reacts to a really strong stimulus of almost any kind by retracting its 

 pseudopodia and assuming a spherical shape. On the other hand a 

 comparatively slight stimulus produces a reaction more or less specific 

 to the particular type of stimulus. As a mechanical stimulus for example 

 one may touch with a needle the edge of the Amoeba on tffie side towards 

 which it is creeping : the Amoeba reacts by turning aside — altering the 

 direction of its movement. Chemical change has been tried on a kind 

 of Amoeba known as Amoeba Umax, characterized by the possession of 

 a single blunt pseudopodium (Fig. 5, A). In this case making the water 

 slightly alkaline was found to be followed by a change in the whole 

 form of the Amoeba which protruded a number of slender tapering 



