280 THE RATE OF GROWTH [ch. 



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 little way towards 

 helping us to comprehend the phenomenon of "multiplication by 

 fission," as it is exemplified 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 as the process rests 

 upon a physical principle, or natural tendency, we may account for 

 its occurrence in a great variety of animals, zoologically dissimilar; 

 and for its presence here and absence there, in forms which are 

 materially different in a physical sense, though zoologically speaking 

 they are very closely allied. 



But the phenomena of regeneration, like all the other phenomena 

 of growth, soon carry us far afield, and we must draw this long 

 discussion to a close. 



* A morphological polarity, or essential difference between one end and the other 

 of a segment, is important even in so simple a case as the internode of a hydroid 

 zoophyte; and an electrical polarity seems always to accompany it. Cf. A. P. 

 Matthews, Amer. Journ. Physiology, viii, j). 294, 1903; E. J. Lund, Journ. Exper. 

 Zool. xxxiy, pp. 477-493; xxxvi, pp. 477^94, 1921-22. 



