680 JULIAN S. HUXLEY 
greater when the wound is not covered—a result presumably 
due to the greater stimulation which the unexcised limb-cells 
then receive (Harrison, 1915). 
After a short time a small nodule of cells begins to protrude 
from the body in this region. If the main limb-rudiment is 
completely removed the nodule may grow into a perfect limb. 
When, however, the main limb-rudiment is transplanted 
less than four segments back on the same side, these nodules, 
after growing a longer or shorter time, begin to shrink, and 
eventually disappear altogether. When the limb-rudiment was 
only transplanted one segment back the nodules appeared after 
about four days, but very speedily began to decrease and had 
disappeared after eight days. When the limb was transplanted 
two segments back the nodules continued to increase till the 
fifth or sixth day, and had disappeared by the eleventh day ; 
when the distance of transplantation was three segments, 
nodule-growth continued until the tenth or eleventh day, 
when the ‘nodule’ was almost as large as the transplanted 
limb; but after this, decrease set in, and all nodules eventually 
disappeared, although not until the eighteenth to twentieth 
day. Finally, when the main limb-bud was removed more 
than three segments from its original site, the regenerating 
nodules always developed into a normal appendage, so that 
two limbs were produced from the one original rudiment. 
The cells of the limb-bud constitute an equipotential system, 
as Harrison has shown. It is therefore clear that the mhibiting 
effect exerted by the main transplanted rudiment on the cells 
left at the original site must be due simply to the greater size 
of the former. The strength of this ‘ dominance ’, however, 
also depends upon the distance of the two systems ; and when 
this distance is increased beyond a certain limit, there is 
no longer any inhibitory effect. If we like, we may say that 
the reason why the cells constituting the normal limb-rudiment 
of Amblystoma do not usually form more than one lmb is 
that they occupy such a small area that any one rudiment 
srowing within that area inhibits the growth of any other. 
Detwiler did not investigate the actual mechanism by which 
