10 ROSS G. HARRISON 
the mesoderm would thus be brought to the surface and the ecto- 
derm buried beneath it. The same effect might be obtamed, 
however, by transplanting the mesoderm alone. While the dif- 
ficulties in this procedure are great, they have now been in a large 
measure overcome. The positive experiments are too few in 
number to warrant an}' very definite statement, but they do indi- 
cate that it is immaterial which surface of the mesodermal disc 
faces laterally. 
A much greater variety of experiments could l3e had by experi- 
menting with positions intermediate between the upright and 
inverted positions, i.e., with limb buds turned, say, 90° instead 
of 180°. Such experiments maj^ jdeld ver}- interesting results, 
but as yet there has not been sufficient time to carrj- them out, 
nor has the efTect of implanting the limb exactly in the midline 
been studied.' 
The experiments with superposed buds were made in the same 
way as the above, except that the mesoderm of the host was not 
excised. In the case of half buds, more combinations are pos- 
sible, as described in the section dealing with this group. Both 
here and in the superposition experiments all possible positions 
with regard to the placement of the graft within the limitations 
stated above were experimented with. Relations of harmony and 
disharmony proved to be the same here as in the case of whole 
buds. 
The total number of cases of which records have been kept is 
462. The analysis is based, however, upon the 271 individuals 
which yielded positive results. The identity of the individual 
cases has been maintained by rearing each in a container by 
itself and keeping a separate history of each. These histories 
consist in notes and in sketches made from thne to time directly 
from the living specimens, mostly with the aid of the camera 
lucida. 
In dealuig with so large a mass of material it has of course been 
necessary to select typical cases for presentation, and in order 
not to interrupt the continuity of the general account, the indi- 
vidual histories, as far as given, have been gathered together in 
an appendix. The main body of the paper has been divided in 
