39© PATTERNS AND PROBLEMS OF DEVELOPMENT 



ASYMMETRY AND MIRROR-IMAGING IN RECONSTITUTION 

 OF AMPHIBIAN LIMBS 



A new limb polarity, opposite in direction to the original, may originate 

 in regeneration of amphibian limbs. A piece of limb including the knee 

 and part of the upper and lower leg implanted in a host body by its distal 

 cut end may show regeneration from its free proximal end, but this re- 

 generate gives rise not to parts normally proximal to the level of the prox- 

 imal cut end on which it appears but to the distal portion of a limb, 

 which may include all levels normally distal to the proximal cut end from 

 which it arises. Its polarity is opposite in direction to that of the original 

 piece and evidently independent of it, but the asymmetry is the same as 

 that of the proximal stump. Evidently the asymmetry provides a pattern 

 at the cut end, and this affects or persists in the regenerating tissue ; but 

 there is no such pattern for the polarity of the regenerated part, and this 

 evidently represents a new pattern originating in the bud of new tissue. 

 The regenerated part is a mirror image of the reversed piece except that 

 it develops distal parts absent from the piece.'' Similarly, pieces of am- 

 phibian tails implanted in the dorsal region by their posterior cut ends 

 may become bipolar by regeneration of a new posterior end from the free 

 anterior cut end (Milojevic et Burian, 1926). Here, also, the symmetry 

 of the regenerated posterior end is the same as that of the implanted 

 piece, but the polarities are opposed; consequently, there is mirror-imag- 

 ing, so far as parts are present in the implanted piece. 



Development of limbs following transplantation of limb primordia in 

 amphibia presents a number of interesting problems and an indefinite 

 range of experimental possibilities; a great variety of experiment and an 

 extensive literature have resulted.'^ According to the terminology adopted 

 by Harrison (1921a), transplantation is orthotopic to the normal location 

 of the limb, on either side of the body, or heterotopic to some other loca- 

 tion — for example, the flank, homopleural to the same side of the body 

 as origin, heteropleural to the opposite side. As regards orientation of the 

 transplanted primordium, dorsodorsal position is with dorsal border dor- 

 sal, dorsoventral, with dorsal border ventral with respect to the host. 

 The anteroposterior axis of the transplant coincides in direction with that 



'7 Delia Valle, 1911, 1913; Kurz, 1922a; Graper, 1922; Milojevic et Grbic, 1925. 



'* Harrison, 19210, 19250, and Przibram, 1924, 1927, give numerous references; the general 

 review by Mangold, 1929a, gives a very complete bibliography. See also Swett, 1923, 1926, 

 1927, 1928a, b, c, 1930, 1932, 1937a, b, 1938a, b, c, 1939; Oka, 1934, and references cited by 

 them. Experimental data concern chiefly the forelimb. 



