Regenerated 

 stump 



Regenerated 

 ray 



Size of 

 original tail 



REGENERATION IN STARFISH AND LIZARD 



A starfish can regrow as many as four rays to full size, if the disk remains intact. 

 Many lizards will regrow missing limbs or tails. In the regenerated tail of Ameiva 

 from Dutch Guiana there are no vertebrae 



Growth Substances Biologists have long been puzzled by the various 

 ways in which plants and animals respond to injury. Healing and regenera- 

 tion are obviously adaptive: they help to preserve the Injured or mutilated 

 Individual. But what makes the healing start? One suggestion is that the 

 tissues stopped growing In the first place through die action of the cells 

 upon one another. That is, each cell might still be able to grow and divide 

 but is kept from doing so by the presence of neighboring cells. When cells 

 are broken, however, two conditions are changed: (1) there is now more 

 room for further growth, and (2) injured cells may throw some growth 

 substance Into the surrounding space. 



These new conditions might explain the sudden proliferation of new cells 

 into the space formed by a wound. According to experiments by George 

 Sperti (1900- ) and associates at Cincinnati and by other investigators, 

 Injured cells do produce substances that stimulate cell-division of living cells. 

 From yeast cells and chicken embryo cells killed under suitable conditions, 

 experimenters removed special substances that hasten the growth of injured 

 tissues. Ointments prepared from such materials have been very helpful in 

 healing severe burns and other wounds. Incidentally, these studies have fur- 

 nished some clues to the further Investigation of the causes of cancers. 

 Cancers are ''wild" growths In tissues that had already completed dieir nor- 

 mal development: the Idea is that injured cells Introduce special growth 

 substances and so bring about abnormal proliferation. 



Poisons The idea of poisoning must be very old In the experience of 

 the race. Something taken into the system interferes with comfort or with 

 life. Acids and alkalies obviously injure tissues, as on the skin. Undoubtedly 



230 



