942 CHARLES ZELENY 
the next succeeding stage. At this step, as probably at other 
steps in the ontogenetic process, removal is followed by a repeti- 
tion of the part removed. The stages need not follow in a defi- 
nite prescribed order. A stage may be repeated when the neces- 
sary stimulus is given. In normal ontogeny the first functional 
operculum has a period of existence as such which ends by its 
breaking off near the base. In its place a rudimentary operculum 
is developed, an operculum which is a preliminary stage in a new 
functional, the second of the Hydroides type, which develops only 
after a long period of latency. If, however, the first operculum 
be lost early in its life, there is no skipping of the later phases 
leading up to a normal loss, but the first stages are repeated and 
an operculum like the one removed results. 
3. . Is the removal of the first functional operculum followed by 
regeneration, first as a branchia which only later develops the 
opercular enlargement or is the opercular modification developed 
directly? The opercular modification is developed directly. An 
enlargement at the cut surface of the stalk is evident very soon 
after the operation. There is no trace of the development, first, 
of a pointed end like that of the original branchia, but opercular 
growth proceeds directly at the regenerating surface. There 
seems thus to be no repetition of the ontogenetic branchial stage 
at this regeneration (fig. 4, Ba). 
The observations and experiments on young Hydroides con- 
tribute the following data on the factors controlling asymmetry 
of the animals. 
1. The animal is originally symmetrical and the appearance of 
the asymmetrical structure, always on the left side, can not be 
due to the preponderance of nutrition on one side as a result of 
a larger amount of material on that side. There is further no indi- 
cation that the original asymmetry is due to the character of the 
tubes or the nature of their curvature. 
2. The removal of a functional operculum is not necessarily 
followed by the development of a rudimentary operculum on the 
same side. The larger mass of material. following the removal, 
was on the opposite side of the body. Nevertheless, the func- 
