SYMMETRY IN TRANSPLANTED LIMBS 85 
occur in a large number of cases, then but 5.5 per cent are 
exceptional. 
The behavior of transplanted lunb buds in accordance with 
the above rules indicates that the posture and the asymmetry 
of the limb is determined neither by the limb itself nor by its 
surroundings exclusively, but by an interaction between the two. 
This is best described by the assumption, that in the stages exper- 
imented upon the anteroposterior axial differentiation is already 
determined within the limb bud, while the ventrodorsal axis (prob- 
ably radio-ulnar of the grown limb) is determined by its orien- 
tation with reference to the surrounding tissues of the host (fig. 
135). In a given place a right limb bud upside down thus be- 
haves like a left limb bud right side up and vice versa (fig. 2). 
It is scarcely necessary to point out that this is not a gravity 
effect, for the embryo lies on its side during the period when the 
dorsoventral axis of the limb is determined, 'upside down' being 
used here merely with reference to the cardinal points of the 
embryo itself. 
What the nature of the influence exerted by the organic envi- 
ronment may be, has not been determined. Whether it acts upon 
the intimate structure of the limb bud or directly upon the difTer- 
entiating systems contained therein, without affecting the inti- 
mate structure as a whole, cannot be answered from the present 
data (p. 101). The influence is not sharply localized, for it is 
the same both in the limb region itself and elsewhere along the 
flank of the embryo, so that it is probably an effect of the axial 
differentiation of the tissue elements themselves. It is possible 
that light may be thrown upon this question by transplanting 
the limb bud to the dorsal or to the ventral midline of the embryo. 
F. The mode of representation of symmetric relations in the limh 
rudiment 
The question whether the adult parts are locahzed in the 
germ, forming a mosaic, must be answered in the negative for 
the limb bud, as used in the experiments, i.e., if we consider as 
such a disc of tissue, three and a half somites in diameter, cen- 
tering ventral to the fourth myotome, and leave out of account 
the outlying regions from which certain portions of the shoulder- 
