688 TEXTBOOK OF ZOOLOGY 



Amphibia 



Among the Amphibia, the Caudata are able to regenerate append- 

 ages. Adult Salientia are unable to do so, though they are more 

 primitive in many skeletal characters than the Caudata. A great 

 deal of research pertinent to an understanding of regeneration has 

 been done with amphibian material. 



Salamander larvae frequently snap off each other's legs or gills. 

 Some salamanders will snap off their tails if they are seized. Gills, 

 legs, and tails are regenerated in both larvae and adults, though 

 regeneration is slower in the adult than in the larva. 



Regeneration may be accomplished through the activity of pre- 

 viously highly differentiated cells. When the lens of either a frog 

 or a salamander eye is destroyed, the cells of the iris undergo a loss 

 of pigment, differentiate, and form a different type of structure 

 which develops into a new lens. This process is quite unlike lens 

 formation in the embryo in which the lens is formed from super- 

 ficial ectoderm cells under the influence of the optic cup. 



New legs are formed by the assembly of undifferentiated connec- 

 tive tissue cells beneath the surface of the wound and by dediffer- 

 entiation of muscle and other cells. Such a group of cells is termed 

 a Mastema. The new leg is formed by the proliferation and differ- 

 entiation of the cells of the blastema in a manner similar to the 

 development of the leg in the embryo. 



After the leg of a salamander is cut off, new leg bones, muscle, 

 and other tissues are formed from the blastema over the old stump, 

 not from the bones and muscle of the old stump directly. The cells 

 that form the blastema are assembled from their resting place in 

 nearby tissues. They are not brought together from distant points 

 in the body. Under local influences they are induced to form a 

 new leg. The strictly local nature of this induction has been demon- 

 strated by transplantation experiments. A boneless leg transplanted 

 to the back of a salamander regenerated its proper complement of 

 bones. Split limbs develop partial limbs complete as to structures 

 normally distal to the point of cleavage, producing duplicated limbs. 



Nerves are not essential to regeneration of a new leg, but if they 

 are prevented from growing into it, the regenerated leg is much 

 smaller than is normal. 



