SYMMETRY REVERSAL AND MIRROR-IMAGING 185 



peculiar condition. Oshima's own explanation of the 

 condition is as follows: ''The growth of the normally 

 developing hydrocoele might have been arrested from 

 some cause and the right anterior coelom, to compensate 

 this defective development, produced a new hydrocoele 

 on the right side." To me the condition is simply an 

 exaggeration of that in which bilaterally symmetrical 

 hydrocoeles are produced. In some way, as the result 

 of abnormal growth conditions, the development of the 

 left-hand hydrocoele is inhibited, while the right-hand 

 hydrocoele becomes physiologically isolated and begins 

 to grow before the left has sufEciently recovered to 

 resume development. The right-hand hydrocoele now 

 inhibits the left because it has a higher rate of develop- 

 ment, and is therefore the superior side. Thus we have 

 before us a case of complete symmetry reversal experi- 

 mentally induced. 



A GENERAL THEORY OF THE PHYSIOLOGY 

 OF SYMMETRY REVERSAL 



Any theory of symmetry reversal must be founded 

 upon a correct understanding of the physiological status 

 that exists in a perfectly bilaterally S3anmetrical organ- 

 ism. I look upon such an organism as the resultant of 

 an intimate interplay and co-operation of two systems of 

 primordia which are in focus upon a single median plane, 

 a plane which is equivalent to a sagittal plane of the adult 

 organism. The two antimeric halves are in a sense twin 

 individuals with a common apical plane. So close is 

 the co-operation or integration of the two halves, 

 however, that one half influences the opposite half in 

 such a way that equivalent structures appear in the two 



