GROWTH, AGE, AND DEATH 295 



type of cells. For example, the nerve cells of the human brain do not 

 undergo division; an injury in which some brain cells are destroyed 

 results in a permanent loss of the nerve cells of that particular 

 region. No other type of cell is capable of being transformed into a 

 brain, or a nerve cell. Similarly, human muscle cells from such 

 muscles as those of the arm cannot undergo division and if de- 

 stroyed or removed are not replaced; their place is taken by a type 

 of connective tissue that forms a scar. 



The replacement of a part which has been lost is known as 

 REGENERATION. Ability to regenerate lost parts, in general, decreases 

 from the lower to the higher types of animals, and in general de- 

 creases with the age of the individual animal. But there are many 

 exceptions to this rule and many factors other than position in the 

 scale of animal life and age of the individual are concerned in this 

 ability to regenerate. 



If an amoeba is cut into two parts, the part which contains the 

 nucleus usually will re-form and in time regain its normal size, 

 while the part lacking a nucleus may initiate but never completes 

 regeneration; parts that lack the nucleus or any part of the nucleus 

 always die. This is also true of other Protozoa. One may state it as 

 a general rule that parts of cells that lack nuclear material do not 

 regenerate and live only a brief time. 



The Porifera and Ccelenterata exhibit the most extensive powers 

 of regeneration. If certain species of sponges are cut into small 

 pieces and the tissues pressed through silk bolting cloth, some varie- 

 ties of cells in the resulting minute fragments will assemble and 

 develop into adults. In some coelenterates also, small fragments 

 will regenerate the whole. If the stem region of many species cf 

 hydroids (p. 102) is cut into sections, the feeding and reproductive 

 hydranths having been removed, each section will regenerate into a 

 new individual. Curiously enough, most free-swimming Ccelen- 

 terata have very little or no power to regenerate. Ability to regen- 

 erate is high in some of the free-living flatworms (Platyhelminthes) 



