246 
ZOOLOGY: R. G. HARRISON 
spending to the dorsal border of the wound (dorso-dorsal) , or in the 
inverted position with the ventral border of the disc at the dorsal bor- 
der of the wound (dorso-ventral) . There were thus eight different pos- 
sible combinations, all of which were tried in numerous individual 
cases. Intermediate positions have not yet been experimented with. 
When the limb bud of one side is implanted on the other, it is obviously 
impossible to place it in a normal posture, for when the dorsal and 
"0"- HOM. HET. HET. 
^-D D Y D-D D V 
FIG. 1. 
Diagram showing the eight different operations. The outline of an Amblystoma em- 
bryo in the operating stage is shown above. The circles within it represent the limb bud, 
in the normal (orthotopic) and the abnormal (heterotopic) location. The four circles be- 
low represent the four different ways in which limb buds may be oriented with reference 
to the cardinal points of the embryo; the letters (d, dorsal; v, ventral; a, anterior; and p, 
posterior) within the circles designate the original cardinal points of the transplanted limb, 
those outside the corresponding points of the embryo. The operations are represented 
to be on the right side. R, right limb bud; L, left limb bud; horn., homopleural; hei., 
heteropleural. 
ventral borders correspond respectively the anterior and posterior bor- 
ders are interchanged and vice versa. Eight further combinations are 
formally possible by interchange of internal (medial) and external 
(lateral) surfaces, though they are impracticable, because the meso- 
derm would thereby be placed on the outside. 
The experiments confirm previous ones in showing that the limb bud 
is a self differentiating body, in so far as it may produce a fore limb 
