90 ROSS G. HARRISON 
lateral axis is probably not differentiated, though in the absence 
of sufficient experiments in reversal of this axis, it is better to 
make no definite assumption regarding the intimate structure 
in relation to it. The dorsoventral axis is at most but slightly 
differentiated, and if it is at all, then the differentiation is revers- 
ible.^^ As already pointed out (p. 55), there is some ground 
for the latter assumption, for it has been observed that after 
transplantations in the heteropleural dorsoventral position (the 
harmonic combination in which the dorsoventral but not the 
anteroposterior axis is reversed) the adjustment of the tissues of 
the lunb bud is apparently not mimediate, but involves a time 
factor, probably not entirely accounted for by the effect of the 
operation as such. Whatever the character of this dorsoventral 
differentiation may be, it is nevertheless very slight in comparison 
with the anteroposterior differentiation, which has become irre- 
versible by the tune the stage in question is reached. 
If we could experunent over a wide enough range of stages, 
it should be possible to determine the time limits of the above 
phases of axial differentiation of the lunb rudiment. At present, 
how^ever, there are no data bearing upon the question, for in the 
earliest stages in which the transplantation of limbs has been 
carried out (embryo with wide open medullary folds), as shown 
by Detwiler ('18), the limb bud follows in its development the 
same rules as here formulated. 
Lest the foregoing scheme seem too formal, it may be pointed 
out that the model has been chosen to explain solely the relatively 
smiple characters of polarity and symmetry. Upon this as a 
basis, further experunentation may yield facts from which the 
mode of representation of more specific form features may be 
determined. There is nothing in such a scheme inconsistent 
with the fact that the cell itself is not a homogeneous system, for 
the model is supposed to represent only that constituent of the 
system which determines the adult character in question. 
8* This is perhaps odd in view of the facts brought out by Przibram ('10 b), 
showing that dorsoventral differentiation is very marked in the animal organiza- 
tion, more so, for instance, than the anteroposterior differentiation. 
