Limb and Girdle 



431 



from above is bowed toward the body. The 

 form changes of the limb are concerned 

 largely with the lengthening of the various 

 segments, notably the digits, and the more 

 distinct demarcation of the arm, forearm and 

 manus. 



Rotation takes place at the shoulder, the 

 arm is directed more laterally and ventrally, 

 so that the tip of the first digit rests on the 

 substrate. Further rotation at this joint, 

 coupled with flexion at the elbow, brings 

 the manus much further forward beneath 

 the gills, and the animal now rests upon two 

 digits of each limb. The balancers, which 

 serve to support the larva, are lost at stage 46. 

 The first muscular movements take place at 

 the shoulder at stage 44, and later, move- 

 ment begins at the elbow (stage 45) and 

 wrist joints (stage 46); the limb is then used 

 in crawling, the positions just described be- 

 ing those at rest. These changes are com- 

 pleted just about the time the yolk is entirely 

 gone and the larva begins to feed (Harri- 

 son, '18). 



BASIC EXPERIMENTAL WORK ON THE 

 LIMB 



The limb bud of the anuran larva con- 

 stitutes a self-differentiating system which 

 develops into a normal limb when trans- 

 planted to new and strange surroundings 

 (Braus, '03, '04b, '09; Banchi, '04, '05; Har- 

 rison, '07). The various problems which 

 have evolved from this fact of amphibian 

 development have become so numerous and 

 so widespread that they form overlapping 

 sequences in modern experimentation. 



The review of only part of the limb prob- 

 lem can be attempted here. This centers 

 about the problems of limb development, 

 limb specificity, limb and girdle relation- 

 ships and reduplication. These taken to- 

 gether form a combination of studies which 

 have to do with the axial relations, their 

 development, determination and behavior. 

 Chronologically, the first problems to be 

 attacked were problems of the outgrowth of 

 the nerve fibers. Their results yielded the 

 facts which later formed the basis for the 

 modern concepts of nerve outgrowth (Harri- 

 son, '15, '24). 



We may now attempt to locate more 

 precisely the borders and surfaces of the 

 definitive limb in the disc of tissue out of 

 which it develops. Assuming them to be in 

 the same relative position at the stage at 

 which the operations are done as later, when 

 these features first become visible, an oblique 



line crossing the disc through the postero- 

 dorsal and the anteroventral quadrants would 

 pass through the ulnar and radial borders 

 and hence divide the flexor from the extensor 

 surface. 



The only reliable information bearing on 

 this question is that given in Swett's ('23) 

 paper. The posterodorsal sector forms the 

 whole flexor surface and the distal portion of 

 the extensor; the anterodorsal sector forms the 

 rest of the extensor surface and extends along 

 the radial border almost to the tip. The 

 anteroventral portion forms the shoulder, 

 while the posterior sector does not partici- 

 pate in the formation of the free limb at all 

 but enters only into the shoulder and body 

 wall. If instead of dividing the limb bud 

 vertically and horizontally, as in Swett's 

 experiments, one were to split it obliquely at 

 the proper inclination, the cut would no 

 doubt pass through the growing point of 

 the limb and separate the flexor from the 

 extensor surfaces. 



Balfour's (1878) discovery of the composite 

 nature of the elasmobranch fin raised also 

 the question of the possible relationship be- 

 tween the mesodermal somites and the de- 

 veloping limb. That this relationship is 

 variable is shown by the work of Harrison 

 (1895) on teleosts, where a complete series 

 of fin structures either dependent or inde- 

 pendent of the mesoderm can be obtained. 

 The limb will develop normally after the 

 damage or complete extirpation of the meso- 

 dermal somites. There is no demonstrable 

 contribution of the somites to the muscula- 

 ture of the limb in amphibians (Byrnes, 

 1898; Lewis, '10; Detwiler, '18, '29). 



Byrnes' (1898) analysis of the situation in 

 the frog embryo was obtained by burning 

 the somites in the hind limb region with a 

 hot needle. The defects in the somites pro- 

 duced no defect in the development or muscu- 

 lature of the limb. This experiment was 

 repeated upon the urodele embryo by Lewis 

 ('10), using a more refined technique. He 

 extirpated the somites, cutting out the meso- 

 derm and observing the subsequent defects 

 in the larva. While the ventrolateral muscu- 

 lature was deficient, the limb developed 

 normally, showing no defects in skeleton or 

 musculature. Detwiler ('18, '29), in his 

 study of girdle formation and also in his 

 spinal cord studies, has performed identical 

 experiments with the same results — per- 

 fectly independent limb development. 



The forelimb region of the urodele embryo 

 is a self-differentiating system. The materials 

 which later form the limb can be located 



