Wound Reparation—A ctinian Tentacles IQI 
considerably nearer the outer surface. Granting that ectoderm 
and entoderm are equally rigid layers, then a contraction of these 
muscle processes would bend the body wall so that the ectoderm 
would be everywhere on the concave side. ‘The observed facts 
show just the opposite conditions—the ectoderm is always on 
the convex side. An explanation of these form changes on the 
ground of contraction of the muscle processes would necessarily 
involve the assumption that the ectoderm is more rigid than the 
entoderm, an assumption which may be regarded as consistent 
with the fact that the entoderm cells are highly vacuolated. But 
we may equally well abandon the muscle explanation entirely and 
suppose that the inbending of the cut edges and the spiral curling 
of the longitudinal half-column of Hydra are due to the existence 
of different degrees of tension in the two body layers. ‘This 
explanation would, of course, apply also to the initial inbending 
of the cut edges of a transversely cut column. Loeb (’g1) de- 
scribes the inbending of cut edges in Cerianthus and suggests that 
it is due to unequal tension in the body layers. Child (’o4a) 
gives an extended account of the behavior of cut edges in Cerian- 
thus. He regards the initial inbending of cut edges as due to 
differences in the elasticity of the body layers, and points out his 
reasons for thinking that the mesoglea is the chief agent in caus- 
ing the bending of the edges. 
Some further considerations seem to me to argue against the 
view that muscle contraction is the only or chief factor in the clos- 
ing of transversely cut edges of Hydra. Except for the initial 
inbending, which is somewhat abrupt, the process consists of a 
very slow and gradual extension of the body wall across the open 
end, and requires for its completion from fifteen minutes to an 
hour. The absolute motion here is incomparably slower than that 
involved in any of those ordinary activities of the normal polyp 
which we are safe in regarding as of muscular character. Again, 
the fact that the closing is complete offers difficulties to the muscle 
explanation. If muscular contraction plays a part in the process, 
it must be accompanied by a certain amount of rearrangement 
of the elements of the tissues, in order that the result may be a 
complete closing and that the two body-layers over the closed 
