Ill] REGENERATION, OR GROWTH AND REPAIR 151 



the possibility of the uninjured Hmb growing all the faster for 

 a time after the animal has been relieved of the other. From the 

 time of amputation, say of ^, ^ begins to grow from zero, with 

 a high "regenerative" velocity; while B, starting from a definite 

 magnitude, continues to increase, with its normal or perhaps 

 somewhat accelerated velocity. The ratio between the two 

 velocities of growth will determine whether, by a given time, 

 A has equalled, outstripped, or still fallen short of the magnitude 

 of i5. 



That this is the gist of the whole problem is confirmed (if 

 confirmation be necessary) by certain experiments of Wilson's. 

 It is known that by section of the nerve to a crab's claw, its 

 growth is retarded, and as the general growth of the animal 

 proceeds the claw comes to appear stunted or dwarfed. Now in 

 such a case as that of Alpheus, we have seen that the rate of 

 regenerative growth in an amputated large claw fails to let it 

 reach or overtake the magnitude of the growing little claw : 

 which latter, in short, now appears as the big one. But if at the 

 same time as we amputate the big claw we also sever the nerve 

 to the lesser one, we so far slow down the latter's growth that 

 the other is able to make up to it, and in this case the two claws 

 continue to grow at approximately equal rates, or in other words 

 continue of coequal size. 



The phenomenon of regeneration goes some way towards 

 helping us to comprehend the phenomenon of "multiplication by 

 fission," as it is exemplified at least in its simpler cases in many 

 worms and worm-like animals. For physical reasons which we 

 shall have to study in another chapter, there is a natural tendency 

 for any tube, if it have the properties of a fluid or semi-fluid 

 substance, to break up into segments after it comes to a certain 

 length ; and nothing can prevent its doing so, except the presence 

 of some controlling force, such for instance as may be due to the 

 pressure of some external support, or some superficial thickening 

 or other intrinsic rigidity of its own substance. If we add to this 

 natural tendency towards fission of a cylindrical or tubular worm, 

 the ordinary phenomenon of regeneration, we have all that is 

 essentially implied in "reproduction by fission." And in so far 



