94 MANUAL OF PHYSIOLOGY. 



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, arid, 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 in 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 can also discriminate between 

 different objects, for we see it greedily flowing around some food 

 material, whilst it carefully avoids other substances with which 

 it comes in contact. 



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

 window, they will be found to cluster on 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 toward 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 bursaria}, 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- 

 cesses (vibratile cilia), which constantly move in a certain direc- 



