6 ROSS G. HARRISON 
ing the diameter of three and a half somites (ca. 0.9 mm.) and 
ventral to the third, fourth, fifth, and half of the sixth myotome, 
is made, and the disc of tissue, including both mesoderm and 
ectoderm, is lifted, after which the remaining mesoderm cells are 
carefully cleaned off. This may all be done without injury to 
the pronephros, which lies immediately dorsal to the lunb rudi- 
ment, though if this organ is injured or even extirpated, there is 
no noticeable effect on the subsequent development of the limb. 
The embryo thus prepared is held in readiness for the grafting, 
being secured in position by pieces of silver wire or glass rod bent 
into proper shape. The limb bud which is to be transplanted is 
removed from another embryo, as described above, care being 
taken in lifting it from its bed to take all of the mesoderm pos- 
sible. It is then transferred on the point of the scissors to the 
first embryo and fitted into the wound. The orientation of the 
bud is important and may be carried out as desired by noting 
the position of pigment markings in the ectoderm of the graft. 
After it has been properly placed, it is held in position for an 
hour or more by a single piece of glass rod, bent into such shape 
that it straddles the embryo, exerting a light pressure upon the 
grafted tissue. The healing of the wound takes place readily, 
though frequently a small area of underlying yolk may be left 
exposed on the border of the wound. This usually heals in a day 
or two and does not seem to influence the result of the experi- 
ment, unless the yolk begins to disintegrate, in which case death 
of the embryo usually follows. 
At the tune when the first experiments were made, the condi- 
tions necessary to prevent regeneration had not been determined, 
so that in a number of cases the extirpated area was too small 
or the wound bed was insufficiently cleaned of scattered meso- 
derm cells for the result to be conclusive. These experiments 
have not been included in the tabulations, but will be referred to 
separately in so far as they are of special interest. In all of the 
later experiments the size and character of the wound in the host 
was such as to preclude regeneration of the limb from that 
source; the resulting limbs must, therefore, have arisen from 
the engrafted tissue. Even in the cases where the wound was 
