GROWTH AND REGENERATION 



269 



we should certainly never expect to make two whole pigs by 

 cutting one into two parts. 1 



Among lobsters, crabs, and crayfish the power of regenera- 

 tion is also present in a very high degree. When one of these 

 animals has his claw or one of 

 the legs caught or mutilated, 

 he may throw the limb off com- 

 pletely, the separation taking 

 place along a definite plane 

 between two of the joints. The 

 wound does not bleed and the 

 lost limb is soon replaced by 

 the regeneration of another 

 from the tissues about the scar. 



322. Vegetative propagation. 

 A stem separated from the 

 root can regenerate roots, as we 

 have already learned (p. 48). 

 Fruit growers propagate new 

 lots of individuals, from trees 

 that are especially desirable, by 

 setting out slips, or cuttings, 

 of these trees and having them 

 " set " roots. In this way all the good qualities of a given 

 tree can be indefinitely extended to a large number of trees. 

 Indeed, we may consider all the cuttings from a single tree as 

 really parts of the tree growing separately. This relation has 

 been described as a discontinuous growth of a single individual. 



1 The growth of the hair or of the finger nails after the ends have been cut 

 off does not represent a case of regeneration. The hair and the nails grow 

 continuously, the live cells in the follicle and in the root of the nail producing 

 new cells which are being pushed forward by the new cells underneath. Cut- 

 ting hair or nails simply removes the external, dead portion of the structure. 

 The new teeth that a child gets after losing the first teeth do not represent 

 regeneration either, since the rudiments of the second teeth are present long 

 before the first teeth drop out. The new teeth are independent structures 

 that develop normally and actually push out the first teeth. 



FIG. 107. Regeneration in starfish 



The mutilation of starfish does not seem 

 to kill them, for each part may regrow 

 enough to complete a new individual. 

 The regenerated ray shown in the figure 

 is smaller than the rest ; but in time the 

 new ray would become full size, since re- 

 generating tissues and organs grow faster 

 than the uninjured parts 



