CONTROL OF ASYMMETRY IN HYDROIDES 943 
tional operculum developed: from the cut surface. A small 
opercular knob at the end of the branchia of the opposite side 
was, however, developed as a result of the operation. 
3. The. result of the operation was an animal with the same 
symmetry as a normal individual. 
SUMMARY 
1. The removal of the first operculum of Hydroides dianthus 
in its early stages before the development of a rudimentary oper- 
culum is followed: 
A. By the regeneration of a new functional operculum of the 
same type as the one removed. There is no reversal of opercula 
such as occurs in the adult. 
B. (1). The-branchia occupying the position of the future 
rudimentary operculum and which in normal development shows 
no opercular modification develops an opercular enlargement at 
its end as a result of the operation. 
(2). The opercular enlargement is developed from cells which 
in normal development do not form an operculum. 
(3). The enlargement is of the rudimentary type such as is 
found in adults of the genus Apomatus. 
2. The regenerated functional operculum is like the one re- 
moved. Regeneration does not go directly to the next succeeding 
or Hydroides type of the operculum. On the other hand neither 
is there a repetition of the preceding stage, the opercular enlarge- 
ment appearing directly without the interposition of a branchial 
tip, later to be modified into an opercular structure. 
3. The results point to the conclusion that reversal of opercula 
in the adult is dependent upon the presence of a specialized struc- 
ture, the rudimentary operculum, capable of developing rapidly 
into a functional operculum. In the absence of such a special 
structure the regenerating tissue on the old functional operculum 
side retains its supremacy, getting an earlier start and inhibiting 
the development of a similar structure on the opposite side. The 
injury to the functional operculum does, however, initiate an 
