86 
ROSS G. HARRISON 
girdle develop. This conclusion is based upon the following- 
evidence derived from the experiments: 1) After extirpation of 
any half of the limb bud, a complete normal limb may develop 
from the remaining half; 2) fusion of two limb buds by super- 
position is followed, if the combination is harmonic, by the devel- 
opment of a single normal limb, which at first is usually larger 
than normal, but in which there is rapid regulation of size; 3) 
HOM.DD 
HOM.DV 
HET.DV 
Fig. 135 Diagram to show determination of asymmetry of limb. The circles 
represent the limb bud, the squares the surrounding part of the embryo. A,D, 
P,V, the cardinal points of the embryo — anterior, dorsal, posterior, and ventral, 
respectively. The heavy arrows represent the determining axes, i.e., the antero- 
posterior axis of the bud and the dorsoventral axis of the surrounding parts; 
UL, future ulnar border; DI, approximate direction of outgrowth. The smaller 
arrows show the other a.xes of bud and surroundings, respectively, which are not 
effective in determining the axes of the definitive limb. 
