THE PHENOMENA OF LIFE 



object-glass under proper conditions, a colorless blood corpuscle contracts 

 and ceases its ameboid movement. 



c. Nerve Influence. By stimulation of the nerves of the frog's cornea, 

 contraction of certain of its branched cells has been produced. 



d. Chemical Stimuli. Water generally stops ameboid movement, and by 

 imbibition causes great swelling and finally bursting of the cells. In some 

 cases, however (myxomycetes), protoplasm can be almost entirely dried up, 

 but remains capable of renewing its movements when again moistened. 

 Dilute salt solution and many dilute acids and alkalies stimulate the move- 

 ments temporarily. Strong acids or alkalies permanently stop the move- 

 ments; ether, chloroform, veratrum, and quinine also stop it for a time. 



Movement is suspended in an atmosphere of hydrogen or carbonic acid 

 and resumed on the admission of air or oxygen, but complete withdrawal of 

 oxygen will after a time kill the protoplasm. 



e. Electrical. Weak currents stimulate protoplasmic movement, while 



strong currents cause the cells to assume a 

 spherical form and to become motionless. 



The Power of Digestion, Respiration, and 

 Nutrition. This consists in the power which 

 is possessed by the ameba and similar animal 

 cells of taking in food, modifying it, building 

 up tissue by assimilating it, and rejecting what 

 is not assimilated. These various processes 

 are effected in some one-celled animals by the 

 protoplasm simply flowing around and en- 

 closing within itself minute organisms such as 

 diatoms and the like. From these it extracts 

 what it requires, and then rejects or excretes the 

 remainder, which has never formed part of the 

 body. This latter proceeding is done by the 

 cell withdrawing itself from the material to be 

 excreted. The assimilation constantly taking 

 place in the body of the ameba is for the 

 purpose of replacing waste of its tissue conse- 

 quent upon manifestation of energy. The 

 respiratory process of absorbing oxygen goes 

 on at the same time. 



The processes which take place in cells, both animal and vegetable, are 

 summed up under the term metabolism (from ^era/foVr;, change). The 

 changes which go on are of two kinds, viz., assimilation, or building up, and 

 dis assimilation, or breaking down; they may be also called, using the nomen- 

 clature of Gaskell, anabolism or constructive metabolism, and catabolism or 

 destructive metabolism. In the direction of anabolism two processes occur, 



-.d 



FIG. 6. Cells from the Stam- 

 inal Hairs of Tradescantia. A, 

 Fresh in water; B, the same cell 

 after slight electrical stimula- 

 tion; a, b, region stimulation; 

 c, d, clumps and knobs of con- 

 tracted protoplasm. (Kiihne.) 



