192 Herbert W. Rand 
end should retain uniformly their original thickness. Further- 
more, if the cut end of the column of Hydra is crushed so that the 
wound is an extremely ragged one with partially detached frag- 
ments of ectoderm and entoderm lying together in a confused 
mass, there takes place a slow and orderly segregation of the mate- 
rial of the two body layers. Ectoderm flows into ectoderm, ento- 
derm into entoderm; everywhere the entoderm retreats to a deep 
position while the ectoderm seeks a superficial one. The con- 
fusion of the two layers is corrected, the raggedness is smoothed 
over, and the result is a closed end with the two body-layers in 
normal relation to each other and of approximately normal thick- 
ness. Explanation of these activities by means of muscular con- 
traction alone can hardly be imagined. Upon the other hand, 
there is every appearance of amceboid motion of the tissue ele- 
ments. 
Numerous experiments, such as those of Driesch (’95) upon the 
echinoderm blastula and gastrula, show that closing of wounds 
regularly takes place in embryonic tissues where muscle fibers 
or processes certainly do not exist. Driesch cut into fragments 
of various sizes the blastula and gastrula of Spherechinus and 
Asterias. In fragments including one-half or more of the original 
forms the wounds promptly closed. 
While the total evidence as to the mechanics of wound closing 
in Hydra is not conclusive, I am of the opinion that. muscular 
contraction plays at most only a part in the process. ‘The initial 
abrupt inbending of the cut edges is doubtless due either to muscle 
contraction or to some preéxistent difference in the elasticity or 
tension of the body layers, but in the complete closing of the wound 
a very important role, if not a leading one, is taken by something 
akin to amceboid cell motion. 
. In amphibian larve, as shown by Born (’96), extensive losses 
of epidermis are repaired in the course of an hour by means of a 
concentric advance of the epidermis from all sides toward the cen- 
ter of the wound, and without increase in the number of cells. 
There are no muscle fibers concerned in this process. Born 
found no evidence of amceboid migration of individual cells. He 
regarded the advance of the epidermis as resulting from a state of 
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