MUTILATION AND REGENERATION 385 



the proliferated cells of the sheath and is not a new 

 growth from the old axis cylinder. 



It is thus seen that mammals have very slight powers 

 of regeneration, though some evidences of the new for- 

 mation of important tissue elements are to be found in 

 most cases of " simple healing." 



IV. Lost Viscera are Regenerated. There can be little 

 doubt but that complexity of organization plays an 

 important part in this event, for the more complexly the 

 organism is constructed, the greater is the mutual 

 dependence and indispensability of its organs. 



To an organism with scarcely any viscera, those it pos- 

 sesses may not be so essential that life may not be easily 

 maintained for a considerable time without them, thus 

 affording opportunity for new organs to form. Such a 

 condition is seen, for example, in certain sea-cucumbers, 

 or holothurians, which when roughly handled eviscerate 

 themselves, yet live on and subsequently regenerate a 

 new set of organs. Were it not for the fact that the 

 holothurian can live for a long time without organs, it 

 could not recover from the injury. In all cases in which 

 an organ is found to regenerate brain of the snail, eye 

 of the newt, etc. the animal must be able to dispense 

 with that organ during as long a time as its regeneration 

 necessitates. 



Less careful attention seems to have been devoted to 

 this phase of regeneration, probably because of the 

 greater difficulty of operating upon the internal organs 

 of small animals. 



Among the vertebrates there is very little true regen- 

 eration of the internal organs. If a kidney be removed, 

 the animal lives on and the other kidney continues to 

 functionate for both, increasing in size for the purpose, 

 not by the formation of new glomerules, but by hyper- 

 trophy, or increase in the size of those already present. 

 In cases in which a kidney is damaged, by operation or 

 disease, new tubules have been found to bud out from 

 the pre-existing tubules and to extend for a considerable 

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