98 ROSS G. HARRISON 
limb buds and of regenerating limbs, which, when it occurs, is 
always combined with reversal of one member? The first visible 
sign of reduplication both in the embryonic limbs and in the re- 
generating blastema is the presence of two growth centers for the 
limb in place of one; each becomes an apex of growth, with a 
resulting bifurcation of the appendage as a whole. The question 
arises whether the doubling of the growth center is antecedent 
to or resultant from the reversal of the asymmetry. From the 
fact that mere mechanical division of a simple regenerating cen- 
ter^"^ may bring about doubling, it would seem to be more prob- 
able, if not certain, that the existence of two growth centers 
within spheres of mutual influence is the factor that produces 
the reversal in one — the one that is less advantageously placed, 
or in which differentiation is less advanced. 
The problem before us thus resolves itself into two phases: 
that of division or repetition of parts and that of symmetry. 
This was clearly seen by Bateson, who has emphasized the funda- 
mental nature of the power to divide. ^'^^ No attempt will be 
made here to analyze this phase of the question. The symmetric 
relations of the repeated parts are, however, so definite and of 
such general recurrence that they, too, are beyond question of a 
fundamental nature. 
The phenomenon of reversal of asymmetry has been treated 
by many investigators as one with that of axial heteromorphosis, 
and yet this is not strictly correct, for the reversal of asymmetry 
may be brought about by the interchange of the poles of any one 
of the three axes to which the object is referred, and not neces- 
sarily the one along which regeneration and differentiation is 
taking place. This is true not only when regeneiation occurs in 
a proxmiodistal direction, as in the cases of Tornier, Zeleny, and 
others, cited above, but also when it takes place distoproximall}-, 
as shown in the two experiments reported b}' Kurz ('12).i°3 
i"Cf. Tornier ('97), Przibram ('02), Reed ('04), Zeleny ('05), Megusar ('07). 
108 "This power to divide is a fundamental attribute of life and of that power 
cell division is a special example." (Problems of Genetics, p. 38.) 
1"^ In somewhat similar experiments by Morgan ('08 a), only the bone, not the 
soft parts, was reversed. Nothing is said regarding the exact character of the 
limbs regenerated. 
