434 ROSS G. HARRISON 
some cases give rise to a limb. Some of the individual dis- 
‘erepancies in this series of experiments may be due to uncon- 
scious varying of this factor. 
In experiments in which another limb bud is transplanted into 
the place of an extirpated normal bud, the grafted tissue must act 
like a piece of body wall from an indifferent origin in so far as 
its effect upon the movement of the cells of the host is concerned. 
It must prevent these cells from wandering into the proper 
position to form a limb, and hence when transplantations of the 
limb bud are undertaken with proper precautions as to size 
of wound and thorough cleaning of the mesoderm, it is safe 
to assume that the limb that does develop arises from the trans- 
planted material and not from the tissues of the host. The 
exact determination of the size and character of the wound neces- 
sary to prevent regeneration is therefore important for the 
proper interpretation of any experiments in which the normal 
limb bud is replaced by a grafted one. 
EFFECT OF REMOVAL OF PORTIONS OF THE LIMB BUD 
It was scarcely to be expected that an organ having such 
marked regenerative capacity as the limb rudiment would show 
any distinct localized effect of the removal of definite portions. 
A number of experiments have nevertheless been made to test 
the prospective potency of its parts. The procedure was as 
follows: The limb area was first bisected by a vertical or a 
horizontal incision and half of the dise—anterior or posterior, 
dorsal or ventral—was removed. Some of the wounds were 
left without further treatment; in others the mesoderm was 
carefully cleaned off and the wound left to heal; and in still 
others the denuded area was covered with ectoderm from the 
flank of another embryo just as in the experiments with whole 
limb buds. In a smaller number of cases, a small circular area, 
1 or 14 somites in diameter, was removed from the center of the 
limb rudiment. The first experiments, which were referred to 
in the preliminary paper (Harrison, ’15), were few in number 
and were made without special cleaning of the wound. ‘They 
resulted in the development of normal or only slightly defective 
