i82 THE PHYSIOLOGY OF TWINNING 



on the left-hand side, it seems quite evident that there 

 is a pronounced superiority of the left side, for the left 

 side develops many important structures that normally 

 do not appear on the right side at all, but are certainly 

 potentially represented on that side. 



We are driven to the conclusion that the appearance 

 and rapid development of the hydrocoele structures on 

 the left side inhibits in some way the development of 

 equivalent structures oh the right. It is well known that 

 an actively growing region may inhibit the development 

 of other actively growing regions, as when a terminal 

 bud in a plant inhibits for some distance back of it the 

 growth of lateral buds. If, however, such a terminal 

 bud is cut off, or if in some way its rate of growth is 

 retarded, the lateral buds are free to go ahead. In some 

 such way as this I would interpret the physiology of 

 the bilaterally symmetrical condition in the starfish 

 larvae above described. Normally the left side, especially 

 in the region where the hydrocoele develops, has a more 

 rapid rate of development than has a similar region on 

 the right side. The left side may be compared, therefore, 

 to a terminal bud which inhibits the development of 

 equivalent growths on the right side. If, however, the 

 developmental rate of the embryo is checked at some 

 critical period when the asymmetry of the two sides is 

 being established, the discrepancy between the two sides 

 fails to appear and they start their hydrocoele dif- 

 ferentiation simultaneously as twin hydro coeles, neither 

 of which is dominant over the other and therefore neither 

 one is inhibited. 



McBride (19 18) succeeded in producing a considerable 

 number of bilaterally symmetrical larvae of Echinus 



