PAEAMCECIUM. 85 



performance of these vegetative functions proceeds until the 

 maximum size is attained, and further nutritive activity is then 

 devoted to reproduction. When growth ceases, commonly the 

 cell divides and forms two distinct individuals. The movements 

 which form the most striking operations of the amoeba are the 

 same as those which take place in protoplasm, except that they 

 are more rapid and obvious. The clear outer layer first flows out 

 as a bud-like process, and, as it is gradually enlarging, some of 

 the central granular part of the cell suddenly tumbles into its 

 midst, where it remains, while other pseudopodia are being thrown 

 out iu the neighborhood, and the same changes repeated in them. 

 It is difficult to watch the motions of an amoeba without being 

 impressed with the idea that it is not only endowed with sensibility 

 but that it also can discriminate between different objects, for we 

 see it greedily flowing around some food material, whilst it care- 

 fully avoids other substances with which it comes in contact. 



If a glass vessel, containing several amcebse, be placed in a 

 window, they will be found to cluster oa the side of the glass most 

 exposed to the light. From this it would appear that, in some 

 obscure way, protoplasm can appreciate light, and respond to its 

 influence by moving towards it. 



This single-celled animal or nucleated mass of protoplasm 

 can perform all the functions of a higher animal. It can move 

 from place to place and assimilate nutriment, apparently discrim- 

 inating between different materials. It distributes nutrient stuffs 

 and oxygen throughout its body by a kind of tissue circulation, 

 and it can appreciate and respond to the most delicate form of 

 stimulus, namely, light, which subtle motion has no effect on the 

 sensory nerve-fibres of the higher animals. 



In some unicellular animals certain parts of the cell are spe- 

 cially modified for the performance of special functions, a divi- 

 sion of labor thus taking place which insures the more perfect 

 accomplishment of the different kinds of activity. In one of the 

 commonest of the Infusoria (Paramcecia bursaricf), which swarm 

 in dirty water, this is well exemplified. The outer layer of the 

 flattened body is denser, and forms a kind of fibrillated corticu- 

 lar case (ectosarc), which is covered over with hair-like pro- 



