Limb and Girdle 



433 



region is removed but the wound not cleaned 

 of mesoderm the amount of regeneration is 

 82 per cent. When, however, the wound is 

 cleaned and a piece of indifferent ectoderm 

 from any part of the body is healed over 

 the wound, there is no regeneration of the 

 limb. The transplantation of indifferent tis- 

 sue has blocked the inwandering of the sur- 

 rounding tissues and prevents reconstitu- 

 tion. If the ectoderm of the normal limb re- 

 gion be thoroughly freed from the closely 

 adherent mesoderm cells, it reacts as indif- 

 ferent ectoderm taken from other regions 

 of the body and prevents limb growth. 



The material which forms the limb con- 

 stitutes an equipotential system; any part 

 can form the whole; two superimposed limb 

 discs can form a normal limb (Harrison, 

 '18, '21; Schwind, '31). Limb equipotenti- 

 ality is based upon the results of experiments 

 in which (1) after the extirpation of any 

 half of the limb bud, the remaining half 

 gives rise to a complete normal limb; (2) 

 two superimposed buds form a limb which 

 at first is large but which rapidly regulates 

 to the normal size; (3) a normal limb may 

 develop from two ventral or two dorsal 

 halves if they are properly oriented; (4) 

 after inversion of the limb disc, the radial 

 portion of the limb gives rise to the ulnar 

 portion and vice versa, changing the pro- 

 spective significance of practically the entire 

 cellular constituency of the bud; (5) the 

 inoculation of mesoderm from the limb 

 region, even thovigh disorganized by the 

 operation, can give rise to a normal limb; 

 (6) a composite limb formed of a half-limb 

 disc of an Amblystoma tigrinum embryo 

 transplanted upon an A. punctatum embryo 

 in place of a half of the A. punctatum limb 

 region gives rise to a perfect composite limb 

 with morphological characteristics of each 

 species recognizable. The evidence seems 

 overwhelming for an equipotential system 

 in Driesch's ('05) sense. 



The limb-forming materials are localized 

 in the mesoderm. This is shown by experi- 

 ments in which the limb ectoderm is trans- 

 planted but does not develop a limb; the 

 mesoderm of the limb disc may be removed 

 and the normal limb ectoderm left in its 

 normal location, but no limb develops; the 

 transplantation of mesoderm alone to a 

 strange environment produces limb develop- 

 ment (Harrison, '18, '25). 



The experimental proof for these state- 

 ments is so rigorous that one would hardlv 

 exnect anv question to be raised with re- 

 gard to the truth of the location. However, 



Filatow ('28) reports that there is no develop- 

 ment of a forelimb in the axolotl after the 

 transplantation of mesoderm alone. "In 20ige 

 Fallen ist das Mesenchymtransplantat nach 

 einigen Tagen verschwunden und dement- 

 sprechend hat sich auch die aussere Vor- 

 wolbung des das Transplantat bedeckenden 

 Epithels vollstandig geglattet." In two other 

 cases, the formation of cartilages unre- 

 lated to the extremity was found. In these 

 cases, the transplanted material remained 

 for a longer time. In all others there was no 

 histological trace of the transplant or struc- 

 tures influenced. From these facts, Filatow 

 considers that the materials transplanted 

 have not yet been determined as limb form- 

 ing. Unfortunately, the exact stage of the ani- 

 mal is not given, but the period of develop- 

 ment utilized extends from the tail-bud 

 stage through early motile stages, in which 

 one would expect, on the basis of Harrison's 

 work, that determination would have taken 

 place. Harrison's work receives confirma- 

 tion from Ruud's ('26) analysis of sym- 

 metry relation, but unfortunately this work 

 does not employ the mesoderm alone so that 

 we have no definite experiments which cor- 

 respond to those of Filatow. Balinsky ('31), 

 working vipon Triturus taeniatus, finds that 

 the mesenchyme plays an important part in 

 limb differentiation. The results are con- 

 flicting and the experiments are not strictly 

 comparable, since the limb and its mesoderm 

 develop much earlier in A. punctatum than 

 in either the axolotl or Triturus. 



Steiner ('21) has published a short series 

 of experiments in which he has seared the 

 epithelial covering of the hindlimb bud of 

 Rana. His conclvision is that the formative 

 influence of the ectoderm acts upon the 

 mesoderm in the production of a limb and 

 that in this the amphibians studied are simi- 

 lar to the higher vertebrates. 



The mass of evidence points to the deter- 

 mination of the limb mesoderm of urodeles 

 as the positive factor of limb formation. The 

 evidence so far amassed for the anuran 

 grovip, which is scattered and scanty, points 

 to an interaction between the ectoderm and 

 mesoderm. Development of the limb is in 

 all cases taken at later stages than the 

 urodele observations and absolutely no criti- 

 cal experiments have been carried through 

 to show the sequence which is hypothesized 

 as causative. Until clear-cut experiments 

 can give more light upon anuran develop- 

 ment, it would seem -wiser to continue the 

 concept of mesodermal determination as a 

 working hypothesis; it is a simpler view. 



