EXCEPTION TO BATESON's RULE OF SECONDARY SYMMETRY. 8 1 



transplantations in which the limb buds were inverted, a certain 

 percentage gave rise to double or twin limbs, one being a 

 mirror image of the other. In a few cases there were still further 

 duplications so that more or less complete triple limbs resulted, 

 having approximately the same relations as found by Bateson in 

 the Arthropoda. 



In the lobster just described, two facts were noted; first, the 

 extra parts are double and second, their pigmentation is reversed 

 indicating an inversion of the double portion of the triple appen- 

 dage. Furthermore, it is generally conceded that most abnormal 

 and duplicated appendages among Crustacea are the result of 

 regenerative processes. Both Reed ( '04) and Emmel ( '07) have 

 found that abnormalities can be produced experimentally by 

 mutilating either the proximal stump or the developing limb bud. 

 It is conceivable then that the triplication found in the present 

 instance may be due to regeneration following injury. In the 

 course of regeneration the growing but may have been also 

 injured so as to cause the development of an extra process 

 (Emmel, '07, pp. 114-115), and this process may have had its 

 dorso-ventral orientation reversed either at the time its develop- 

 ment was initiated or at some later date. 



Keeping the above statements in mind it therefore becomes 

 possible to elaborate a more or less satisfactory hypothetical 

 explanation of the triple chela. First, there may have been an 

 injury to a normally developed crushing claw, followed by autot- 

 omy. Later the developing bud may have been injured, resul- 

 ting in the appearance of an extra bud on the surface of the 

 primary one. As a consequence of the injury, the primary bud 

 did not develop as a crushing claw but as a small "nipper." 

 (Sufficient evidence has been adduced by Emmel in 1907, Figs. 

 24 to 31, to prove that abnormal symmetrical claws in lobsters 

 do arise through regeneration following mutilation.) The 

 extra bud, due to mechanical displacement of its tissue at the time 

 of the first injury, or as a result of a subsequent accident, has 

 had its dorso-ventral orientation reversed, causing the develop- 

 ment of twin crushing claws which are mirror images of each 

 other. 



