Early 20th century Danish physiologist August Krogh once said, "for a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied" (Krebs 1975).  In the field of developmental biology, the axolotl  (Ambystoma mexicanum) epitomizes the type of animal that Krogh eluded to. 

     Axolotls are attractive subjects in the field of developmental biology for several reasons.  They are known to be hardy, quick to mature, breed well in captivity, and have large embryos that are easily manipulated (Smith 1989).  In addition, a broad range of pigment patterns makes the axolotl the focus of several developmental and molecular analyses (Shaffer 1993). 

     Another reason that the axolotl is considered the "animal of choice" for developmental studies, is its paedomorphic condition: retaining its ancestral juvenile characteristics as a descendant adult species (Shaffer 1993).  Although paedomorphosis is observed in other amphibians, A. mexicanum is one of the most famous examples due to its virtual abandonment of metamorphosis.  In fact, there is a long and distinguished history of research pertaining to endocrine control and the morphological effects of paedomorphosis in axolotls (Shaffer 1993).