REGENERATION 13 



I have referred to the slight power o£ regeneration possessed by 

 the blind Proteus in regard to its legs or tail, and I connected this 

 with the absence of enemies in its thinly peopled cave-habitat. But 

 the same animal can regenerate its gills when these are bitten 

 off, and this is probably associated with the habit that Proteus 

 has, in common with other newts with external gills, of nibbling 

 at its neighbour's gills. Thus, the power of regenerating the 

 gills was retained even when the animals migrated to the quiet 

 caves of Krain, and were thus secured from the attacks of other 

 enemies. 



In lizards, a leg which has been cut off does not grow again, but 

 an amputated tail does, and this has quite a definite biological reason, 

 since the active little animal will seldom be caught by the foot by 

 any pursuer, but may easily be caught by the tail, which is far 

 behind. Thus the tail is adapted not only for regenei-ation, but 

 also for ' autotomy,' that is, for breaking off easily when it is caught 

 hold of. 



We have already seen that some segmented worms have a very 

 high regenerative capacity ; yet every part cannot produce every 

 other, and while, in LuTnbricidus, any piece of from five to nine 

 segments is able to grow a new head or tail, neither ten nor twenty 

 nor all the segments together, if they are halved longitudinally, can 

 reproduce the other half, and the cause of this inability does not lie in 

 the fact that the animal is thereby hindered from taking food, for even 

 the transversely cut pieces do not feed until they have grown a new 

 head and tail. The reason must lie in the fact that the primary con- 

 stituents for this kind of regeneration are wanting, and they are so 

 because a longitudinal splitting of this cylindrical and relatively thin 

 animal never occurs under natural conditions, and thus could not be 

 provided against by Nature ^. 



That regeneration of this kind could have been arranged for if it 

 had been useful we learn from the Planarians among the flat worms, 

 in which every piece cut out of the body, large or very small, from the 

 middle, from the left side, or from the right side of the animal, grows 

 into a complete Planarian. The animal can be halved longitudinally, as 

 in Fig. 97, and each half will grow to a whole. This again is quite 

 intelligible from the biological point of view, for these flat, soft, 



' Morgan maintains that this statement is incorrect, and that Lumh-iculus is 

 capable of lateral regeneration. But if we look into the matter more closely we find 

 that all he says is, that small gaps made by cutting a piece out of one side are filled 

 up again, while the cut pieces perish. If the whole animal be halved, according to 

 Morgan, both halves die, or if a 'very long piece ' be cut out of one side, not only this 

 piece dies, but also 'the remaining piece.' There is thus, as I have said, an essential 

 difference between the regenerative capacity of Lumbrimhis and that of Planaria. 



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