Section VII 



CHAPTER 4 



Limb and Girdle 



J. S. NICHOLAS 



VERTEBRATE APPENDAGES 



Vertebrate appendages ai'e outgrowths of 

 materials located in the body wall. The 

 localization and determination of the con- 

 stituent elements occur early in the history 

 of the embryo. Morphologically, there are 

 commonly distinguished two borders, the 

 anterior or preaxial and the posterior or 

 postaxial, and two surfaces, dorsal and ven- 

 tral, which will later become respectively 

 the extensor and flexor surfaces of the free 

 appendage. 



In the embryo, an ectodermal fin fold 

 clearly marks the pre- and postaxial borders. 

 The pectoral and pelvic fins of fishes and 

 the extremities of all tetrapod vertebrates are 

 rotated away from the primitive embryonic 

 position during development. 



Frequently, the pre- and postaxial borders 

 are not readily distingviishable in the devel- 

 oping tetrapod limb. This is particularly 

 so in amphibians where there is no ecto- 

 dermal fold, the limb developing as a cylin- 

 drical outgrowth with a rounded tip. In the 

 lizards, an ectodermal fold is present (Mol- 

 lier, 1895; Peter, '03; Braus, '04a). 



The fold when first observed is longitudi- 

 nal, the preaxial portion being anterior and 

 the postaxial portion being posterior. Dur- 

 ing the course of development, there is a tor- 

 sion in the forelimb which brings the 

 preaxial or radial border ventrally while the 

 postaxial or ulnar border becomes dorsal in 

 position. This process is reversed in the 

 hind limb. 



Since the ectodermal fold is lacking in 

 Amblystoma the pre- and postaxial borders 

 are first recognizable in the forelimb when 

 they have undergone a partial rotation about 

 halfway between the assumed original rota- 

 tion and the final degree of limb torsion. 

 The distal portion of the extremity later des- 

 tined to form the hand serves as an index 

 of the torsion. The plane of flattening of the 

 hand is inclined 45 degrees to the horizontal 



plane of the embryo. Because of the late 

 appearance of the limb borders, it is im- 

 possible to ascertain by observation which 

 radius of the limb disc represents the future 

 radial border. It is indicated, however, in 

 Swett's ('23) work that the material which 

 later is distributed along the proximal border 

 is found in the anteroventral quadrant of 

 the limb disc not far from the center and 

 about a radius which lies from 30 to 45 de- 

 grees anterior to the dorsoventral axis. The 

 materials composing the limb bud of Am- 

 blystoma are localized much nearer to their 

 definitive location than they are in the lizard. 

 This seems to be the case in even the earliest 

 limb transplantations (Detwiler, '29). This 

 means that as the materials grow into the 

 free extremity they develop without torsion 

 into a limb which is apparently partially 

 twisted. After the digits have formed, further 

 torsion takes place, turning the radial border 

 to a ventral position and the ulnar to a 

 dorsal one. 



THE DEVELOPMENT OF THE FORELIMB 

 IN AMBLYSTOMA' 



In the beginning tail-bud stage (Fig. 152^, 

 stage 25) the pronephric swelling is visible 

 and the somites may be observed through the 

 ectoderm. There is no distinct limb bud 

 present, but the region centered under the 

 fourth segment just ventral to the pronephros 

 contains the material that will give rise to 

 the limb. When the tail bud is more marked 

 (Fig. 152B, stage 29) the somatopleure ven- 

 tral to the pronephros is thickened. The 

 material is, however, a region rather than 

 a "limb bud" on the surface of the embryo, 

 since the chief cause of the swelling in that 

 region is the pronephric swelling, the somato- 

 pleural thickening merely serving to round 



* Modified from Harrison's ('18) description of 

 the course of normal development of the forelimb 

 of the spotted salamander, A. punctatum. 



429 



