482 



BIOLOGY AND HUMAN LIFE 



When a lobster, a crab, or a crayfish has a claw or a leg 

 caught or mutilated, it throws the limb off completely, the 

 separation taking place along a definite plane between two of 



the joints, and then gradually grows 

 a new appendage to take the place of 

 the lost one. A single ray of a star- 

 fish will be regenerated, or even two 

 or three rays (see Fig. 201). Sala- 

 manders regenerate tails and legs, 

 and the triton, one of the lizards, can 

 regenerate the eye. In general, how- 

 ever, the more highly specialized or- 

 gans are not readily regenerated, and 

 the highest animals do not regenerate 

 lost parts as do the lower animals. 

 A human finger when cut off will not 

 be regenerated, although the stump 

 will heal ; and we should certainly 

 not expect to get two whole pigs by 

 cutting one into two parts. 



Under certain conditions the tissues of 

 one of the higher animals may begin to 

 form masses of cells through cell division 

 after full growth has been attained, and this 

 leads to serious disturbances. The disease 

 cancer is due to an abnormal growth whose 

 causes are not understood. 



353. Grafting in animals. Grafting 

 is possible in animals, but in unequal de- 

 gree among the various classes. In the 

 insects experimental grafts have been 

 produced by using two halves from two 

 different individuals. From a practical 

 point of view the most interesting are the 

 grafts of skin from one person to another. In recent 3'ears many experi- 

 ments have been made in grafting pieces of bone, sections of arteries, 

 and even whole kidneys and other organs, from one animal to another. 

 By experimenting upon dogs and other lower vertebrates with the same 



a 



Fig. 200. Regeneration in 

 earthworms 



a, worm from which the anterior 

 end had been cut off; h, worm 

 from which the posterior end 

 had been cut off. The dotted 

 lines show where the cut was 

 made. The shaded portions rep- 

 resent the new growths. (After 

 Morgan) 



