ZOOLOGY: R, G. HARRISON 
249 
(corresponding to its new surroundings) ; at the same time the original 
bud became reduced to a small spur or appendage upon the other. 
Rule 3 modified; 6 cases (18.2%). 
4. Heteropleural dorso-ventral transplantations developed into: (a) 
Single limbs of reversed laterality somewhat retarded in their develop- 
ment. Rule 2; 17 cases (94.4%). {h) DupHcate limbs. Rule 3; a 
single case only (5.5%). 
The results of the experiments are summed up in diagrammatic 
form in figure 2. The ulnar border of the limb is at first always dor- 
sal slightly inclined to medial, as in the normal appendage, while the 
palmar surface is medial slightly inclined to ventral, but the limb may 
point in either an anterior or a posterior direction. Bending of the 
elbow or rotation from the shoulder joint may of course modify the 
positions later in development. The reversal of laterality which takes 
place is as if the limbs were mirrored in either one of two planes, one of 
which is horizontal and the other vertical with respect to the main 
axis of the embryo. When the reversal takes place across the former 
plane it is complete and no reduplication occurs. It is accomplished 
immediately or at least very early in development, so that there is little 
or no outward sign of disturbance and development is but slightly re- 
tarded. This plane may be called the primary one. It comes into 
play when the dorso-ventral axis of the transplanted bud is inverted. 
When mirroring takes place in the vertical plane, which may be called 
the secondary one, actual reduplications are found. This occurs in 
those combinations in which the dorso-ventral axis of the disc is not 
inverted and it may also, occur secondarily following reversal across 
the primary plane as just described. The twinning of the appendages 
may, however, be masked, at least in orthotopic transplantations, 
through the absorption of the original (primary) limb bud (3 c). 
The relations of the duplicate limbs change considerably during 
development, and their definitive position is subject to considerable 
variation. There may be further reduplication, so that more or less 
complete triple limbs may result. The three limbs then have approxi- 
mately the same relations as found by Bateson,^^ especially in arthropods. 
In two of the combinations the limbs that develop are in normal orien- 
tation with respect to the cardinal points of the embryo. One of these 
(hom. dd.) is when the graft is placed in the original normal posture, 
the other (het. dv.) when it is placed upside down on the opposite side 
of the body. These- combinations may be termed harmonic. The 
other two combinations, the simply inverted bud (hom. dv.) and the 
bud planted on the opposite side of the body in upright position (het. 
