40 ROSS G. HARRISON 
panied by reduplication in the second. In the latter the second- 
ary bud, being the mirror image of the other, is again reversed 
back to the original prospective asymmetry of the transplanted 
bud. This then occupies an approximately normal position and 
may function to a considerable extent as a normal limb, though 
impeded by the connection with its mate. These two results are 
directly comparable to those of the heterotopic transplantations 
of the corresponding class (fig. 2). 
The third result, in which normal homopleural limbs develop, 
reaching their normal position gradually by rotation, is funda- 
mentally different. Here no reversal occurs; the limb bud begins 
its development as a self-differentiating system, but later, under 
the stress of the changed relation to its environment, it comes 
again into normal posture. 
What determines whether the limb bud shall reverse its asym- 
metry or rotate back to its noimal posture? The earlier experi- 
ments of this series afforded no satisfactory answer to this ques- 
tion. It was certainly not due to the size of the wound, the 
mode of preparation of the wound, the presence or absence of 
the pronephros, or the age of the embryo. What seemed most 
likely was that there were minor accidental differences in the 
amount of rotation to which the limb bud was subjected at the 
time of operation. It was conceivable, for instance, that if the 
disc were rotated anteriorly around the dorsal semicircumference 
of the wound a little less than 180° (fig. 60), the reversing effect 
of its organic environment might be lessened and the rotation 
back to normal position facilitated; in this case a normal non- 
reversed limb would result. If, on the other hand, the grafted 
bud were rotated 180° or slightly more, the reversing effect might 
be at a maximum and rotation most impeded, in which case a 
heteropleural limb or twin limbs would arise. 
Experiments made in the spring of 1917 had for their main 
purpose the testing of this hypothesis. Operations were done 
in pairs; in one case the limb disc was rotated about the dorsal 
circumference in a posteroanterior direction slightly more than 
180° and in the other slightly less, extremes being probably not 
more than 190° and 170°, respectively (see histories on pp. 125-6). 
