Wound Reparation—A ctinian Tentacles ZIT 
been made upon a large number of animals and representatives 
of various groups—chiefly ccelenterates, worms and vertebrates— 
agree as to the following results. If a portion, not unreasonably 
large, of the surface of the animal is denuded of its normal ecto- 
dermal layer, the lost outer layer is quickly replaced through the 
agency of the neighboring ectoderm. ‘The process certainly does 
not depend in any way upon contraction of muscle cells, it is prob- 
ably not under nerve control, and it does not necessarily involve 
cell division. It consists rather of a rearrangement of existing 
ectoderm cells, a concentric centripetal movement of the edges 
of the layer—in the earthworm, at least, an independent migra- 
tory activity of epidermal cells. Similarly, if the entodermal 
cavity 1s exposed 1 its walls tend to close upon themselves and assume 
a deep position with reference to the ectoderm. In more complex 
organisms, such as annelids, this may be wholly or partly due to 
the action of an investing muscle layer. ‘The significant fact is that 
the same thing happens in such a simple animal as Hydra where 
the muscle processes can hardly be accountable for it, and—still 
more significant—in two-layered embryos. Thus, the two- 
layered condition having once been established, it is then the 
primary function of an ectoderm cell to be on the outside. More 
than that, it is the function of ectoderm cells collectively to cover 
all outside surface. The denuding of a bit of surface of its proper 
layer of ectoderm appears to demonstrate a special affinity of 
ectoderm cells for the exposed surface of deep tissues. 
The normal perforations of the body wall arising by the com- 
monly observed processes of invagination, evagination and rupture 
are determined we do not as yet know how. But artificial per- 
forations of the body wall are usually promptly closed primarily 
through this capacity of the body-layer elements for positional 
readjustment. This capacity is inherent in the tissues. It does 
not matter, therefore, whether the fragment of organism be a 
major one (the actinian minus a bit of tentacle) or a minor one 
(the bit of tentacle which has been cut off), for in both cases the 
body layers set about the structural repair of the breach in the 
same way. 
In all of these reparative readjustments the elemental process 
appears to be of the nature of amoeboid motion. 
