THE WONDER OF LIFE 573 



many similar cases which' suggest that the result is, as it 

 were, a compromise between the inherent growth-tendencies 

 of the organism and the environmental stimuh operative in 

 each case. 



Theoretical Considerations. — In what way is it 

 possible for us to imagine the regenerative capacity of 

 organisms ? A crocodile loses a tooth, but beneath its 

 hollow base there lay another which now fills the gap. 

 Not far off there is a rudiment of another, and so on. The 

 adder casts a fang, but there is another ready to shp into 

 its place, and to re-estabhsh in a very interesting way the 

 connection with the poison duct. Not far ofi there is a 

 rudiment of another fang, and so on. But when a crab 

 loses a claw there is no under-study lying ready at the area 

 of rupture. There is no rudiment of a visible nature, and 

 yet the regeneration is duly effected. How can we con- 

 ceive of this ? 



In certain cases, as in plants, there is visible evidence 

 of persisting embryonic tissue — the cambium — which 

 has retained the formative capacity of the original fertihzed 

 egg-ceU. In many of the lower animals, such as polyps, 

 division of labour has not gone very far, and there are 

 visible ' interstitial cells ' which have remained undifferen- 

 tiated and might be compared to the cambiimi cells of 

 plants. But in most of the cases which we have discussed 

 in this chapter regeneration takes place from amid a 

 stump of differentiated cells. In some instances there is 

 an apparent undoing of the differentiation, a return to a 

 simpler type, and a multiphcation at a embryonic level. 

 Sometimes considerable assistance appears to be afforded 

 by migrant amcEboid cells of the body — the phagocytes — 

 which appear on the scene of the accident and are very 



