72 PROBLEMS OF GENETICS 



wound. Some of the best known of these instances are presented 

 by the paired extra appendages of Insects and Crustacea. Some 

 years ago I made an examination of all the examples of such 

 monstrosities to which access was to be obtained, and it was with 

 no ordinary feeling of excitement that I found that these super- 

 numerary structures were commonly disposed on a recognizable 

 geometrical plan, having definite spatial relations both to each 

 other and to the normal limb from which they grew. The more 

 recent researches of Tornier 7 and especially his experiments on 

 the Frog have shown that a cut into the posterior limb-bud 

 induces the outgrowth of such a pair of limbs at the wounded 

 place. Few observations can compare with this in novelty or 

 significance; and though we cannot yet interpret these phenomena 

 or place them in their proper relations with normal occurrences, 

 we feel convinced that here is an observation which is no mere 

 isolated curiosity but a discovery destined to throw a new light 

 on biological mechanics. The supernumerary legs of the Frog 

 are evidently grouped in a system of symmetry similar to that 

 which those of the Arthropods exhibit, and though in Arthropods 

 paired repetitions have not been actually produced by injury 

 under experimental conditions we need now have no hesitation 

 in referring them to these causes as Przibram has done. 



At this point some of the special features of the super- 

 numerary appendages become important. First they may arise 

 at any point on the normal limb, being found in all situations 

 from the base to the apex. Nor are they limited as to the surface 

 from which they spring, arising sometimes from the dorsal, 

 anterior, ventral, or posterior surfaces, or at points intermediate 

 between these principal surfaces. 



With rare and dubious exceptions, the parts which are con- 

 tained in these extra appendages are only those which lie periph- 

 eral to their point of origin. Thus when the point of origin is 

 in the apical joint of the tarsus, the extra growth if completely 

 developed consists of a double tarsal apex bearing two pairs 

 of claws. If they arise from the tibia, two complete tarsi are 



* Arch. /. Entwm., XX, 1905, p. 76; Sitzungsb. d. Ges. Naturf., Berlin, 1907, 

 p. 41, etc. 



