558 THE WONDER OF LIFE 



experimental embryology which show that a cleavage-cell 

 or blastomere may have no small amount of residual power 

 beyond that which it normally expresses ; (2) the normal 

 process of tissue-regeneration which makes good the every- 

 day wear and tear ; (3) the frequency of asexual reproduc- 

 tion by buds or by fission ; and (4) those cases where a 

 minute fragment re-grows the whole. 



Unequal Distribution of Regenerative Capacity. — 

 One of the significant facts regarding the power of re-grow- 

 ing lost parts is its unequal distribution among the various 

 types of animals. It is very common among worm-types, 

 but almost absent in Nematodes — perhaps because a rupture 

 of the body in these worms is rapidly fatal. It is common 

 among Chaetopods, but there is not much of it in leeches — 

 partly because the shppery surface and tough body- 

 wall make these animals but little hable to injury. It is 

 very general among Arthropods, where legs are so hable 

 to breakage, but there is not much of it among Molluscs, 

 perhaps because most of these are shut up in shells. It is 

 well seen in Amphibians, especially among tadpoles and 

 newts, but it is not much in evidence among fishes. It is 

 common among Hzards, but there is httle of it among 

 snakes. A bird's toe or the end of a mammal's tail can 

 hardly be regarded as more complex than a starfish's arm 

 or the visceral organs inside a feather-star's calyx, but while 

 regeneration is exceedingly characteristic of Echinoderms, 

 it is at a minimum in Birds and Mammals. 



AVhen we follow the inequahty of distribution into greater 

 detail it becomes at once more striking and more luminous. 

 It begins to reveal its significance. In many famihes of 

 hzards it is the rule that a lost tail is regenerated, but in 

 those hzards which strike with their tails, Werner has 



