152 BIOLOGY 



cells that make up such organisms are not independent and can- 

 not live long except when related to the other cells that make up 

 the multicellular organism. Nevertheless, there is a certain 

 amount of independence in the individual cells, especially among 

 plants and some of the lowest animals; for in these we may 

 remove only a comparatively small number of cells from the 

 whole organism and these cells will still retain their vitality, still 

 continue their power of growth, and under proper circumstances 

 develop more cells which eventually become exactly like the 

 animal from which they were obtained. This is especially true 

 of Hydra, which can be cut into many pieces, each piece retain- 

 ing the power of independent life, and in time becoming an inde- 

 pendent and well-developed animal. In such low organisms the 

 life of the organism as a complex has not wholly destroyed the 

 independence of the individual parts. This is more or less true 

 throughout the whole of the plant kingdom. Among most higher 

 plants as well as the lower, small pieces separated from the 

 parent plant will not die at once, but may, if put under proper 

 conditions, develop into fully grown individuals like those from 

 which the fragments were obtained. 



With animals, however, it is only among the lowest and 

 simplest forms that a piece, containing a relatively small 

 number of cells, can be separated from the rest and still be 

 capable of developing into a new organism like the original, as 

 in the case of the Hydra. As we pass to the higher animals 

 this power of regeneration disappears, and among almost all 

 animals, even of comparatively low structure, the independent 

 life of the parts is lost, so that when one portion is removed 

 from the complex that makes up the animal it no longer retains 

 its power of life and growth. But even in these cases and among 

 the highest animals, we do find that some parts may have more 

 or less independent life when separated from the organism of 

 which they are a part. In an animal like a frog, for example, 

 the heart may be totally removed from the body and it will still 

 keep up its life for many hours when put under proper condi- 



