Z ROSS G. HARRISON 
General discussion S3 
E. The rules of symmetry 83 
F. The mode of representation of symmetric relations in the limb 
rudiment 85 
G. Reduplication and the problem of polarity and heteromorphosis. 92 
H. Form regulation and function in transplanted limbs 102 
General summary 109 
List of references 115 
Appendix — ^Histories of selected individual cases 119 
INTRODUCTION 
The circumstance that origmally suggested the present study 
was the apparent difference in the results obtained b}^ Streeter 
('07) and by Spemann ('10) in their respective experiments with 
the amphibian ear vesicle. According to the original account 
of Streeter, the otocyst, when taken out of an embryo just after 
closure and replaced after having been rotated 180° on any of 
its axes, develops in normal posture, though a right ^^esicle placed 
on the left side remains true to its side of origin. According to 
Spemann, the inverted vesicle develops in inverted position, the 
rudiments of the constituent parts being localized, at the time 
of operation. Although subsequent work by Streeter ('14) seems 
to have shown that the normal development of the inverted 
vesicles, found in his cases, was due to their regaining normal 
posture by rotation as a whole, the original divergence of results 
nevertheless had raised theoretical questions of great interest, 
which Spemann has ably discussed. The main question was 
whether we might have in the otic vesicle an 'harmonic equipo- 
tential system' with its future asymmetry in some way stamped 
upon its intimate structure. Though Spemann 's analysis an- 
swers the question in the negative, as far as the closed ear vesicle 
is concerned, it is nevertheless important to determine how far, 
if at all, systems of this kind are present in the embryo, for their 
study would throw light upon the question of the mode of rep- 
resentation of adult form characters in the germ, giving evidence 
from a new quarter with regard to the great problems of devel- 
opment which have usually been approached by way of experi- 
ments upon the unsegmented egg and the early stages of cleav- 
age. The method of embryonic transplantation obviously 
