THE DEVELOPMENT OF ORGANISMS 753 



and also to the re-growth of a whole from a separated-off fragment. 

 It implies processes of growth and differentiation, outside the 

 normal course of individual development, though some of its most 

 striking instances are to be found in embryos, rather than in adults. 



Notable Instances among Animals. — When a lizard struggles 

 convulsively in a bird's bill, it may reflexly break off its tail, right 

 across a weak plane which traverses the vertebral column, at one 

 place or at several; and it may thus escape. Moreover, the loss 

 implied in the involuntary surrender of a part is not permanent, for 

 the lizard can re-grow a tail. It is true that the re-growth is a make- 

 shift, an unsegmented rod without vertebrae, but it is none the less 

 a tail, and almost as effective as the original in aiding locomotion. 

 There is a formation of new muscles and of new scales; and an 

 interesting point, noted by Boulenger, is that the scales reproduced 

 are sometimes nearer those of an ancestral type. The regeneration 

 is often fairly rapid, thus in the case of some Portuguese geckos 

 which had lost their tails, a re-growth of half an inch — about quarter 

 of the total length, took place in the course of a six weeks' voyage, 

 during which the animals had no food. 



As other well-known instances of organic regeneration among 

 animals, we may mention the wounded earthworm's re-growth of 

 a new tail or even head, the starfish's replacement of a lost arm, the 

 crab's restoration of a lost great claw. So the snail can re-grow its 

 horn over and over again, never omitting the little eye at the tip; 

 certain male cuttlefishes renew the sperm-laden arm which is 

 loosened off into the mantle-cavity of the female; and the newt 

 replaces the terminal part of a leg that has been bitten off by a fish. 

 But while it is interesting to recall such striking cases, we must 

 realise that this regenerative capacity is very widely distributed 

 among animals, especially in the lower forms. Nor can this process 

 be sharply demarcated from the healing of wounds; for though 

 our scars express an imperfect superficial renewal of the skin, the 

 underlying tissues are usually weU-repaired. 



Analogous Phenomena. — Regeneration is not to be thought of 

 as a phenomenon apart. It is linked {a) to the normal process of 

 growth, and to that discontinuous growth or budding which becomes 

 asexual multiplication; {b) to the multiplicative fragmentation of 

 some sea- anemones, worms, and even starfishes; (c) to the continuous 

 replacement of some hard-worked tissues, and to the persistent 

 growth of such structures as a rabbit's front teeth, or our finger- 

 nails; (d) to the normal restoration of parts periodically shed, 

 notably the antlers of stags and the posterior body of Palolo 

 worms; and (e) to the way in which a nucleated fragment of an 

 egg, or a separated-off blast omere, may form a whole embryo. 



Sporadic Distribution of Regeneration. — A survey of the 

 renewals of lost parts throughout the animal kingdom at once shows 



vol. ii c 



