SYMMETRY IN TRANSPLANTED LIMBS . 99 
The fundamental phenomenon, therefore, is not that a particular 
axis is reversed, hut that reversal occurs at all, and how it is 
brought about. 
Organic polarity, in general, has been based either on the sup- 
posed polarization of the organic units themselves or upon a 
supposed gradient of a functional (Child) or material (Morgan, 
'05) nature, running from one end of the organism to the other. 
There is evidence for the occurrence of both factors, and what 
seems most likely is that both are at play. Under certain cir- 
cumstances they act in the same direction; under different condi- 
tions one ma}^ antagonize and retard, or even overcome, the other, 
as seems likely in heteromorphic regeneration where polarity is 
reversed (earthworm, planarians, amphibian tail, etc.). Przi- 
bram ('13), who advocates a theory combining the two factors, 
which he calls 'Richtungspolaritat' and 'Schichtungspolaritiit,' 
respecti\'ely, nevertheless regards the reversal of polarity as due 
to actual rotation of the cells. He ('06, '10 a) cites unpublished 
work by Hadzi in support of this view, and adopts it in his dia- 
grams ('09) illustrating the five fundamental varieties of opera- 
tion leading to regeneration (Biotechnik) . 
The figures are not convincing, however, for just as much rota- 
tion of the cells is shown at the end where polarity is not reversed 
as at the other end where it is reversed (Przibram, '09, pi. XV, 
Ih-Sh), and in fact, as expressed in these diagrams, what turning 
is shown is nothing more than a wound-healing process. Until it 
is demonstrated that rotation of the cells as a whole takes place 
solely in heteromorphic regeneration, it cannot be used to explain 
reversal of polarity. 
So long as the elementary units of the limb bud have one plane 
of symmetry left, and the final asymmetry of the Ibnb remains to 
be determined by its relation to certain axes of the embryo, ^i" it 
11" In the case of asymmetric organisms, the elementary' units, if representing 
the form of the organism at all, must be postulated as asymmetric themselves. 
In the case of paired organs, each asymmetric in itself, but symmetrical with 
respect to its opposite, polarization on the transverse axis may be assumed as 
due to the position of the parts with respect to the other two axes (cf. Przibram, 
'13, p. 38) and not as necessarih- due to actual differentiation of the elements in 
the transverse direction. 
