378 BIOLOGY: GENERAL AND MEDICAL 



that certain of them can regenerate the entire head, 

 collar, or foot. 



Since the time of Spallanzani much experimental 

 work has been done and many facts added to the knowl- 

 edge of the subject, though we are still greatly in need 

 of illumination concerning the general principles by 

 which what is known can be correctly correlated. 



We now know that lizards frequently lose their tails 

 and regenerate them, also that the animals seem to know 

 that they can do so, for when caught they unhesitat- 

 ingly snap them off. to escape. Though it can re- 

 generate the tail, a lizard cannot regenerate the limbs 

 or even the toes. Newts not only regenerate the tail 

 and limbs, but also the eyes. Crustaceans crabs and 

 lobsters regenerate legs, fighting claws, and sometimes 

 antennae and eyes. Certain arthropods myriapods, 

 arachnids, and a few insects are able to regenerate 

 lost limbs, but this power is restricted to a few species of 

 scattered groups. 



In all of these cases certain facts regarding the regen- 

 erative power must be noted. Thus, the extent of the 

 mutilation determines whether the injured animal shall 

 die or live as well as whether the wound shall simply 

 heal or shall regenerate. In speaking of the salamander's 

 legs it has already been remarked that though they may 

 regenerate many times in succession, if the amputation 

 be performed too close to the body, healing without 

 regeneration results. The legs of crabs and lobsters 

 regenerate best from a certain point known as the 

 "breaking-joint," where the legs are constricted and 

 weakened so that when the animal is caught and held it 

 not infrequently frees itself by fracturing the leg at this 

 point. If the leg be broken below the breaking-joint, 

 the animal usually rebreaks it at that point and casts 

 aside the intervening piece. When the leg is amputated 

 above the breaking-joint, it is regenerated with greater 

 difficulty. Centipedes, tarantulas, and walking-stick 

 insects are found to have "breaking-joints," and these 



