CONTROL OF ASYMMETRY IN HYDROIDES 937 
EXPERIMENTS ON ASYMMETRY IN YOUNG INDIVIDUALS 
The first functional operculum appears before the first rudi- 
mentary (fig. 4). During the stage in which the opercular stalk 
retains its branchial filaments there is as yet no modification of the 
branchia of the opposite side. It ends in a point just like that of 
other branchiae. Buds of new pinnules are however absent 
(figs. 3 and 4). There is at this time no perfected mechanism for 
reversal as in the aduit. The removal of the functional opercu- 
lum, or at an earlier stage the removal of the end of the branchia 
which later develops the opercular enlargement, was accomplished 
in several individuals (figs. 2and 4). Ia only a few of them, how- 
ever, was the operation clean cut and‘the further history of the 
experiment followed. 
The character of the material did not allow experimenting with 
narcotization. It was therefore necessary to keep watch of the 
young animals under a binocular microscope, holding a needle 
knife blade above the opening of the tube. With exceptional 
good fortune it was possible to remove the terminal cup ofthe 
operculum in a few instances without seriously injuring the rest 
of the animal, though in most cases the neighboring branchia of 
the same side was also injured. 
There was no reversal of opercula, though interesting develop- 
ments followed. In place of the removed functional operculum 
a new one like the one removed was developed. There was no 
loss of the old stalk, the regeneration taking place directly at the 
cut surface (fig. 4, Ba). It might have been expected that, in 
case regeneration occurred, the new operculum would at once 
grow out as one of the Hydroides type as found after the first 
natural reversal. This, however, did not occur. The regenerated 
structure therefore repeats the stage of the removed structure 
and does not pass on to the next ontogenetic stage (fig. 7, A). 
The effect upon the branchia of the opposite side is also inter- 
esting. The terminal part of the branchia develops a small 
knob, approaching in character a rudiméntary operculum of this 
species but formed from a group of cells which in normal growth 
never develop opercular enlargements, but are, in fact, lost when 
