MUTILATION AND REGENERATION 415 



on to the distal fragment. When these growing axis 

 cylinders are able, as the result of a neurotropic influence, 

 to find their way into or along the old sheaths, the prog- 

 ress toward the completion of the conducting tract is 

 rapid, otherwise it is slow and more complicated and 

 perhaps less perfect in the end. Some observers deny 

 that the distal fragment degenerates completely, but 

 think that, like the proximal end, it only degenerates a 

 short distance so that the growing proximal end need 

 not renew the entire path of conduction, but only so 

 much as has-been destroyed. Others think that the 

 axis cylinder fibre is restored through the activity of 

 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 



