690 TEXTBOOK OF ZOOLOGY 



Regeneration may be prevented by x-rays, or sometimes by simply 

 sewing the edges of the wound together. After application of 

 x-rays, epithelium migrates over the wound surface, but no blastema 

 is formed. 



Reptilia 



Some of the lizards are capable of regenerating new tails. Indeed, 

 some of them have special areas developed where breaking occurs 

 when the tail is seized just as do the legs of the hermit crab. Spheno- 

 don, the most primitive of the reptiles, is capable of regeneration of 

 a new tail. The regenerated tail contains a central cartilage cylinder 

 instead of vertebrae and the regenerated skin and other tissues are 

 not quite normal. None of the reptiles is capable of regenerating 

 new legs. 



Aves 



The most striking regenerative capacity among birds is the abil- 

 ity to grow new feathers. Each feather is subtended by a feather 

 germ, a sort of permanent blastema, from which a new feather is 

 formed if the old one is lost through molt or accident. 



Regeneration of the ovary is quite extensive if operative injury is 

 inflicted in the young chick. If the left ovary of a young female 

 chick is removed, the right, normally rudimentary, grows to form 

 an ovary, an ovotestis, or a testis. Experiments indicate that the 

 medullary portion of the primary gonad gives rise to a testis and 

 that some medullary substance remains in the rudimentary right 

 ovary. When the inhibiting influence of the normally functioning 

 left ovary is removed, this medullary substance may proliferate to 

 form a testis. Such sex-reversed birds are usually sterile. 



Mammalia 



No mammal is capable of growing a new appendage if aji old one 

 is lost nor of completely regenerating any other major organ. Re- 

 generation of tissues, however, is quite general. Hair, hoof, horn, 

 and epithelium regularly regenerate from germinal layers or areas. 

 If these germinal areas are destroyed, regeneration fails. Baldness 

 is, of course, a familiar example of failure of regeneration. 



Regeneration in the group is largely limited to repair rather than 

 replacement. In this, as in the case of agamic reproduction, it is 

 difficult to differentiate between processes which may be classified 



