SYMMETRY IN TRANSPLANTED LIMBS 
89 
the groups become differently modified, then the structure be- 
comes bilaterally symmetrical (fig. 136, 3). And, finally, if three 
become modified, so that all four are different, then the arrange- 
ment becomes asymmetrical (fig. 136, 4 and 5) as in the case of 
optically active substances with an asymmetric carbon atom. In 
the last phase there are two kinds of indi\'iduals, which are exactly 
alike in every respect, except that they are the mirror images of 
^A A 
A A 
LEFT 
RIGHT 
Fig. 136 Diagram to show hypothetical progressive differentiation of the 
structural units. 1) condition of isotropy; 2) polarization with reference to one 
axis; 3) bilateral symmetry (two axes differentiated); 4 and 5) condition of com- 
plete asymmetry (three axes differentiated) giving right and left enantiomorphs. 
one another — in other words, rights and lefts. This is expressed 
in aggregate form in the right- and left-handed crystals corre- 
sponding, respectively, to the dextro- and laevo-rotatory forms 
of otherwise identical substances. 
The experiments with limbs show that the bud at the time of 
transplantation is in either the second or the third phase, prol^ably 
the former. There must be a differentiation along the antero- 
posterior axis, because if this is reversed the limb shows it by 
growing in a direction nearly opposite the normal. The medio- 
