TWINNING (DUPLICITY) IN LIMBS 195 



limb-buds at an early stage to another part of the body. 

 He used a small circular punch by means of which he 

 was able to cut out a little circle of the body wall of the 

 embryo from the region known to contain the primordia 

 of the fore limbs. These little circles of embryonic 

 tissue were then placed in various positions (hind part 

 before, upside down, and diagonally) in wound beds of 

 the correct size that had been cut out of the body wall 

 at other places than those normal for the development 

 of limbs. Sometimes a right-hand limb-bud was put 

 on the left side, and vice versa, in the various positions 

 stated above. These small transplanted limb primordia 

 produce limbs in their new positions, but they show 

 varied results according to the side on which they are 

 transplanted and the position in which the pieces were 

 placed. In general it may be said that a graft tends to 

 produce a limb with the same symmetry relations that 

 it would have had if left where it originally was, but 

 that there is more or less complete symmetry reversal 

 in some cases. The normal limb grows backward, but 

 if a limb-bud is transplanted hind-part-before the limb 

 will grow forward. The palmar surface of the limb 

 tends to form on the side turned toward the body of 

 the animal and the ulnar border tends to be dorsal. 

 Harrison says: 



The above circumstances determine the asymmetry of the 

 limb as follows: when the dorso-ventral axis is not inverted, the 

 original prospective asymmetry persists; when the axis is inverted, 

 the asymmetry is reversed. In more general terms: the asym- 

 metry of the limb is determined by two factors, the polarization 

 of the anterio-posterior axis of the limb-bud and the orientation 

 of the limb-bud with respect to the dorso-ventral polarization of 

 its organic environment. 



