436 ROSS G. HARRISON 
group it is obvious that the proportion of normal limbs varies 
very greatly among the different operations. Thus when the 
ventral half of the limb bud was removed, eight out of ten re- 
sulted in perfect limbs; but when the dorsal half was taken out, 
only one out of twelve gave rise to a normal appendage. The 
anterior and posterior halves occupy an intermediate position 
between the two extremes. These discrepancies are largely, 
if not entirely, due to the difficulty of exactly halving the ma- 
terial that is to form the limb. In bisecting it vertically the 
myotomes were used as a guide, and in most cases the incision 
was made below the middle of the fourth somite, leaving one 
and a half somites in front of the incision and two behind it. 
This seems to divide the material more nearly in half than when 
the incision is made a quarter somite further back. In the case 
of the horizontal incision it is more difficult to divide the rudi- 
ment accurately, because there is no sharply defined landmark. 
The attempt was made to cut a little below the pronephric 
swelling. The results show that more of the limb material lies 
above the cut than below. In other words, if the circular area, 
centering just below the pronephros and extending from the 
boundary between the second and third somites to half way 
through the sixth, is bisected vertically and horizontally, more 
limb-forming material lies dorsal to the line than ventral, and 
more anterior than posterior. The lines shown in figure 2 desig- 
nate more nearly the exact quartering of the material. 
The fact that normal appendages resulted in some cases after 
each kind of operation shows that there is no localization of 
prospective potencies in the operating stage. Examination of 
the character of the defects that do arise confirms this conclusion. 
The large proportion of the latter are defects of the whole limb, 
which remains a nodule or becomes entirely resorbed. They 
must be ascribed to the general effect of the operation and not 
to the removal of any specific material. Seventeen cases out 
of a total of forty-eight operations (not including cases that 
died) with cleaned wounds resulted in this wa'y. Of the partial 
defects, listed under the caption ‘defective,’ all five affected the 
hand, especially the digits. The most marked case was one in 
