68 ROSS G. HARRISON ' 
(figs. 116 and 117). A sixth case" is inconclusive owing to gen- 
eral weakness of embryo, but it is not inconsistent with the 
results in the other cases. A typical history is given in the appen- 
dix (p. 131). 
Though more than the normal amount of material is present 
in the composite limb bud, in two of the cases of this group the 
developing limb is recorded at first as shghtly smaller than the 
normal. In one case no difference in size was noted, while in two 
the limb on the operated side is noted as larger. In the other 
harmonic combination (homopleural dorsodorsal) all cases 
showed the limb on the operated side to be somewhat larger in 
size. 
17. Discussion of experiments with superposed limb buds. The 
principal differences between these experiments and those of 
simple transplantation are that in the former the tissue available 
for the formation of the limb is approximately double in 
amount, and there is a mixture of tissues having two different or- 
ientations except in the one group, homopleural dorsodorsal. In 
the harmonic combinations the amount of tissue is so regulated 
that after a time size-difTerences disappear. The amount of tis- 
sue is, moreover, never quite double that of the normal limb be- 
cause of the material lost by the operation and of the general 
retardation of growth due to the same cause. In the case of 
the heteropleural dorsoventral grafts, which are classed as har- 
monic, some readjustment must be necessary, as shown by the 
amount of retardation. 
In all of the disharmonic combinations there is a mixture of tis- 
sues dilTerently oriented and with difTerent prospective meaning 
as regards the particular asymmetry of the future limb. The 
twin limbs that arise are, therefore, not necessarily due to redu- 
plication by budding, as they must be in the simple transplanta- 
tions, but probably in part at least to the circumstance that one 
of the pair develops out of the original limb bud, while the other 
is from the transplanted tissue. 
" S. E. 16. 
