igS THE PHYSIOLOGY OF TWINNING 



dorsal and the thumb side is ventral, (c) The palm 

 surface of the limb is posterior and the back of the hand 

 is anterior. 



With these points of orientation in mind, we are now 

 in a position to compare and contrast the twinning 

 situation as it presents itself in conjoined twins and in 

 double limbs. In conjoined twins we found that the 

 organic symmetry relations were influenced only by 

 internal factors so that- each bilateral half was situated 

 in a position exactly equivalent to that of the other. 

 In a limb-bud, however, the environmental relations of 

 the body to which the limb-bud belongs clearly exercise 

 an influence upon the symmetry of the limb. If a 

 limb-bud were to be transplanted to the median dorsal 

 region, so that its dorsal half fell on the right and its 

 ventral half on the left of the primary axis of the em- 

 bryo, I suspect there would grow a perfectly bilaterally 

 symmetrical limb with no difference between radial 

 and ulnar sides and no difference between little finger 

 and thumb. Growing as it does, however, the limb-bud 

 cannot be bilaterally symmetrical because the ulnar 

 border of the limb (little-finger side) is dorsal, and 

 therefore has a higher rate of metabolism than has the 

 radial or thumb side. Physiologically the ulnar side is 

 the superior side. There is therefore an asymmetry 

 quite similar to that in the whole body of such animals 

 as the echinoderms. 



If the superiority of the ulnar side should be broken 

 down in any way we might expect to get an equivalence 

 of superior and inferior sides something like that seen 

 in conjoined twins with mirror-image symmetry, or like 

 the starfish larvae with paired hydrocoele structures. 



