204 THE PHYSIOLOGY OF TWINNING 



It seems highly probable that all vertebrates undergo 

 processes of bilateral doubling such as these described. 

 Essentially the sanae conditions have been shown to 

 apply in many cases among the arthropods. These 

 and the vertebrates are the only groups that have well- 

 defined limbs. I am aware that our attempt to effect 

 an analysis of the problem of symmetry reversal is not 

 an unqualified success, but I am convinced that this 

 point of view has elements of value and will help in the 

 ultimate solution of a very perplexing problem. 



THE CAUSES OF LIMB-DOUBLING 



If limb-doubling is equivalent to that of bilateral 

 twinning it should have the same general causes. It is 

 our present theory that limb-doubling is caused by a 

 lowering of the developmental rate at the time when 

 the limb-bud is beginning its process of outgrowth or 

 before the terminal elements have become differentiated. 

 Now what evidences have we at present that limb- 

 doubling is associated with developmental retardation? 

 There are two independent lines of evidence. The first 

 is that double limbs most frequently occur in connection 

 with regeneration processes in which we know that, 

 prior to regrowth, there takes place a nearly complete 

 dedifferentiation of tissues and a loss of axiate organiza- 

 tion. It commonly happens in regeneration experiments 

 that two heads grow out in place of one and similarly 

 the embryonic rudiment of a regenerating limb is likely 

 to lose its unity and become double. Regeneration 

 processes are usually less rapid in their early stages 

 than are normal developmental processes, and require a 

 reaxiation process before they can go ahead. The second 



