810 
ROSS GRANVILLE HARRISON 
entiation, unaccompanied, however, by growth, i.e., increase in 
length. 
The changes which take place through the protoplasmic activity 
of the embryonic cells can usually be distinguished from those 
which are due to the action of the clot or the sudden spreading 
out of the drop of plasma. Likewise the fibrin can readily be dis- 
tinguished from the hyaline protoplasm of the cells, although even 
in the fresh specimen it varies considerably in appearance. Some- 
times the fibrin filaments, in spite of their extreme fineness, are 
plainly visible, and in other cases there are comparatively few to 
be seen. They may be found singly or in bundles, and often run 
for a long distance in a straight line, or sweep around in circles, 
the individual filaments running from one strand to another. The 
threads are seen to radiate from the transplanted tissue, and often 
they may be traced from the hyaline ectoplasm of the embryonic 
cells, upon w^hich they apparently exert considerable tension. 
This may result in drawing out the ectoplasm to a narrow fringe 
(figs. 9, 10, and 28), which differs, however, from the fringe of 
active protoplasm described above, in that it does not continually 
undergo changes in form. Evidence of still greater tension is 
found in cells which are drawn out into spindle shape, and which 
often seem to be pulled along bodily, as maj^ be seen in figs. 9, 10 
and 11 which show three successive views of the same cell {ct2.) 
Sometimes long chains of cells in single file or slightly overlap- 
ping one another may be formed. Direct evidence of mechanical 
tension may be had in observations like the following: A long 
thin fiber-like structure was observed in a preparation containing 
branchial ectoderm extending, tightly stretched, from a pear- 
shaped cell to a mass of cells some distance away, when suddenly 
this strand of protoplasm broke, contracting into a short thick 
process which remained attached to the cell. Again very fine 
protoplasmic threads are frequentl}^ found spanning the round 
openings in the masses of tissues, which have been described 
above (fig. 16^^/.). These threads are always taut and are appar- 
ently due to the stretching of originally shorter protoplasmic 
connections between the cells, as the holes in the tissue enlarge. 
