332 ANIMAL DEVELOPMENT 



of Crustaceans are a particularly good illustration of this. It is 

 no infrequent thing to see a Crab or Lobster in which one pair 

 of pincers is of insignificant size, the fact being that it is a new 

 growth replacing an accidental loss. A creature of this kind 

 which is getting the worst of a combat thinks nothing of aban- 

 doning a few limbs, leaving them in possession of the enemy, 

 and thus securing a safe retreat. 



Among Backboned Animals (Vertebrata) there are some lower 

 types in which vegetative propagation regularly takes place. Of 

 this the best examples are the Sea-Squirts {^Urochorda), many 

 of which produce colonies by means of budding. But in the 

 higher types, from Fishes to Mammals, development from eggs 

 is the invariable rule. Regeneration of lost parts may, however, 

 take place to a varying extent. Among Amphibians (Amphibia), 

 for instance, the limbs of Newts and other tailed forms, as also 

 those of very young individuals of the tailless species (such as 

 Frogs), can be completely re-grown if lost. The best example in 

 the class of Reptiles (Reptilia) is afforded by those lizards which 

 sacrifice their tails for protective reasons (see vol. ii, p. 371). 

 These members afterwards sprout again, but the internal struc- 

 tures are not renewed in their entirety, and the covering of scales 

 may be of different pattern. In the hot-blooded Birds (Aves) 

 and Mammals (Mammalia) the regenerative processes are practi- 

 cally limited to the healing of wounds and the renewal of patches 

 of skin which have been destroyed. It is obvious that the lives 

 of many individuals would be sacrificed but for the possibilities 

 in this direction. And it is a matter of common knowledge that 

 remarkable results have been attained in modern surgery by 

 "grafting" little bits of living healthy skin into the surface of 

 extensive burns and the like. 



