4 BOSS G. HARRISON 
transplantation it was observed that the reversal came al^out 
by a process of reduplication or twinning. By following closely 
the history of individual cases, it became evident that the double 
formation was not infrequently obscured by the preponderance 
of the reduplicating limb bud over the original, so that the former 
grew into a member of opposite asymmetry, while the original 
bud was reduced to a mere spur or nodule, which might readily 
be overlooked. The tendency to produce duplicities thus proved 
to be even greater than the actual number of fully developed 
cases indicated. In other cases the reversal appeared to be more 
direct; at least, a limb of the side of origin often failed to appear 
as such on the surface, though slight irregularities in the early 
stages of development, coupled with an appreciable delay in the 
process, showed that some internal readjustment of the grafted 
tissue was taking place. 
It seemed that the functional activity of the limb, when trans- 
planted to its normal envhonment, might accentuate the appar- 
ently anomalous results just described. In order to eliminate 
this factor, a series of expermients in which the limb bud was 
grafted on some other part of the bod}^ was undertaken. Here 
the proper nervous connections did not become established, func- 
tional activity was usually lacking or was at best but slightly 
developed, and the undisturbed effect, upon development, of the 
relative orientation of the tissues of graft and host could be ob- 
served. The latter experiments led to the formulation of the 
three following simple rules, ^ which hold for implantations either 
in normal or in abnormal location: 
Rule 1. A bud that is not inverted (dorsodorsal) gives rise to 
a limb of the side of origin of the bud, whether implanted on the 
same or on the opposite side of the body. 
Rule 2. An inverted bud (dorsoventral) gives rise to a limb 
of reversed asymmetry, whether implanted on the same or on 
the opposite side of the bod3^ 
Rule 3. When double limbs arise, the original one (the one 
first to begin its development) has its asymmetry fixed in accord- 
^ These rules were phrased somewhat differently in two preliminary communi- 
cations ('17 a and '17 h). 
