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LIMB FIELD OPERATIONS 



GENETIC FACTORS RELATING TO LIMB FORMATION 



A, BATES OF GBOWTH - HETEROGONIC : 



The following experiments are deviaed to demonstrate the growth rate differences in- 

 herent in transplants, especially when made between fast and slow growing species. Up to 

 the beginning of the larval period, when the yolk has been absorbed, the more slowly de- 

 veloping Amblystoma punctatum has a considerable forearm with two digits while at the cor- 

 responding stage of development of Amblystoma tlgrinum (fast developer) and A. mexlcaniun 

 larvae have only mesenchymatous nodules. Shortly after this stage, however, the A. ti- 

 grinum limbs reach and surpass the size of those found in A. punctatum. To demonstrate 

 that the transplanted limb generally maintains its inherent (genetic) rate of growth 

 (Harrison, 19214- ) the following experiments are designed: 



a. Amblystoma tigrinum limb-bud is completely replaced by A. punctatum limb bud at 

 stage #35. 



b. A. punctatum limb bud completely replaced by A. tlgrinum limb bud at stage #55 • 



c. Heterotopic transplants made between A. tigrinum and A. punctatum, with host 

 limb fields untouched. This gives an excellent basis for comparisons. 



Such transpleintations as these. Involving mesoderm alone, would demonstrate most con- 

 clusively the relative functions of ectoderm and mesoderm in limb formation, outlined in 

 the previous set of experiments. The mesoderm of the limb bud seems to control the form, 

 the rate of growth, and the ultimate size of the urodele limb. And in turn, the limb af- 

 fects the number of ganglion cells in the adult spinal ganglia (Schwind, 1952). 



DORSAL VIEW 



VENTRAL VIEW 



HETEROPLASTIC TRANSPLANTATION OF LIMB FROM A. TIGRINUM 

 TO RIGHT SIDE OF A. PUNCTATUM 



B. PIGMENTARY DIFFEEENCES: 



This experiment Is best demonstrated with the white axolotl, A. mexlcanum, in limb 

 transplantations with the highly pigmented A. tigrinum. There are three main types of pig- 

 ment cells; Melanophores, with darker granular pigment; Xanthophores, with yellow lipoid 



