412 PATTERNS AND PROBLEMS OF DEVELOPMENT 



by development of the rudimentary into a functional operculum, and the 

 originally functional one regenerates as rudimentary. When only the rudi- 

 mentary operculum is removed, it regenerates as rudimentary. When 

 both are removed, both may develop the functional form or reversal, or 

 persistence of the original asymmetry may result. Also, when the anterior 

 part of the body is removed in the thoracic region, two opercula of func- 

 tional type develop in the anterior regeneration (Zeleny, 1902, 1905a, 

 1911). 



Many decapod Crustacea show heterochely; that is, one chela is large, 

 the other small; in some species only in one sex; or the two chelae differ 

 in form and function, being distinguished as crushing and pinching or as 

 snapping and cutting. In different species the asymmetry ranges from a 

 high degree of uniformity as regards right-left position of the two types 

 to approximate equality of positions. In some forms asymmetry is not 

 reversed by removal of either claw. In others removal of the large claw 

 is followed by development of the small into a large and by regeneration 

 of a small in place of the original large one. In still others, like the lobster, 

 with two claws differing in type, reversal of asymmetry does not occur in 

 the adult but does in the young animal (Emmel, 1908); and when both 

 claws are removed, even in the adult, both may regenerate as crushers 

 (Emmel, 1906). Experimental reversal of asymmetry apparently has no 

 relation to the constancy of a particular asymmetry in the species. There 

 is no experimental reversal in some forms with approximate equality of 

 right-left positions, or it appears only in young animals.^'* 



In the forms with experimentally reversible asymmetry the subordi- 

 nate member attains dominance when the other is removed; but if both 

 are removed at the same time, both may regenerate as dominant mem- 

 bers, or an asymmetry may result. The experimental results are much 

 like those with hydroid and planarian pieces. Unipolarity or bipolarity 

 results according to the difference or similarity in condition at the two 

 ends. So with the appendages asymmetry represents a difference in con- 

 dition on the two sides of the segment concerned, and experiment shows 

 that one side is dominant, as in unipolar forms; but if both sides begin 

 development at the same time, both may develop as dominant but with- 

 out inhibiting each other, like bipolar forms. 



How the dominance of one appendage becomes effective on the other 

 is not certainly known. Some evidence was obtained by E. B. Wilson 



2^ For further data sec Przibram, 1Q02, 1905, 1907, 1908, 1918; E. B. Wilson, i903(;; 

 Morgan, igo4C, 1924; Zeleny, 1905; Emmel, 1906, 1908. 



