THE CONTINUITY OF LIFE 



235 



plants, and soon after Hydra was discovered experiments were 

 made to determine whether the organism was a plant or an animal. 

 Specimens were cut into several pieces and it was found that each 

 piece developed into a complete Hydra. This result, from the ideas 

 of the time, should have led to the conclusion that Hydra is a plant, 

 but additional characteristics were observed which outweighed all 

 other considerations. Accordingly Hydra was recognized as an ani- 

 mal with the power of replacing lost parts. (Fig. 158.) 



Since the classic work on Hydra the power of regeneration has 

 come to be recognized as a fundamental property of all animals. 

 It is exhibited to the greatest degree among the lower animals, 

 while in the higher Vertebrates it is confined chiefly to the replace- 



Piece 



cut out 



Changes in a 

 piece of normal Hydra 



HYDRA 



Later changes in 

 the same piece 



Fig. 158. — Regeneration in Hydra. 



ment of cells which especially suffer from wear and tear, such as 

 those forming the outer layers of the skin. Regeneration is one 

 phase of a fundamental property of protoplasm, namely growth, 

 whether it consists in restoring a part of a Paramecium, trans- 

 forming a bit of a Flatworm into a complete animal, or replacing 

 half of an Earthworm, the head of a Snail, the claw of a Crayfish, 

 or the leg of a Salamander. But, it will be recognized that associ- 

 ated with growth there are complex processes of simplification 

 (dedifferentiation) of tissues and organs, and later a rebuilding 

 (differentiation) in order that a part may become again a normally 

 organized whole. Witness certain marine Flatworms that can be 

 cut down to less than one two-hundred-thousandth of their origi- 

 nal size and still become miniature worms like the original. 

 The experimental study of regeneration phenomena has opened 



