446 ROSS G. HARRISON 
of a whole organism out of any part of sufficient size. Another 
test, more difficult and less frequently tried, is the rearing of a 
single organism from two eggs or embryos which have been made 
to fuse together." 
The experiments described in the foregoing section have shown 
that any half of the limb bud can give rise to a whole limb. 
Those to be taken up now demonstrate that two limb buds fused 
together will develop into a normal single limb. 
The operation of superimposition or fusing together of two 
limb buds is carried out as follows: A circular incision of the 
proper size (33 segments in diameter) is made through the ecto- 
derm of the fore-limb region, care being taken to injure the 
underlying mesoderm as little as possible. The ectoderm may 
then be readily stripped from the middle layer by inserting the 
points of the scissors or needle -at the dorsal part of the cut 
and tearing the ectoderm away. Often a few mesoderm cells, 
especially from the ventro-posterior quadrant, come off with the 
ectoderm, but the greater part of the cells composing the limb 
bud remain in place and not infrequently every cell is left intact. 
An entire limb bud from another embryo is then grafted in the ~ 
usual way over the mesoderm of the limb thus exposed, and such 
grafts heal in very readily. The results of these experiments 
differ according to the orientation of the grafted bud, in har- 
mony with the rules of laterality (Harrison, 717). At present 
only the cases in which the grafted bud has its normal orienta- 
tion will be considered. 
Five such experiments were made, all of the embryos surviving 
and giving the same results. Normal limbs developed which at 
first showed difference in size, but this difference was after a time 
obliterated. The greater massiveness of the double bud was 
usually apparent the day after operation and was most marked 
about three or four days later. In two cases it is recorded as 
persisting for twelve days, though the difference from the nor- 
mal gradually diminishes, disappearing entirely by the time the 
yolk is entirely gone, i.e., about eighteen days. 
11 Cf. Morgan, ’95; Driesch, ’00, ’10; Goldfarb, ’14. 
