42 WARREN H. LEWIS 
is less, they tend to flatten out in various directions. ‘This is 
often still more marked in the isolated cells near the periphery 
The explanatioy of this is probably not in a longitudinal polarity 
that disappears as the cells move peripheralward, but in the 
interplay of surface tension and of the differential adhesiveness 
of the cells for each other and for the cover-glass. The cells 
have apparently a greater adhesiveness for the cover-glass 
than for each other, and at the periphery, where they have more 
space, they are drawn out more and more on the cover-glass 
by the tension pull of the surface film of the fluid medium which 
lies against the cover-glass, until a balance between capillary 
attraction and cohesion is reached. ‘Tait (18) has recently de- 
scribed the spreading-out of certain blood-cells on glass as due to 
capillary attraction or, in other words, to surface-tension phe- 
nomena. The endothelial cells are evidently sticky and semifluid 
and are subject to the same physical forces as the blood-cells 
described by Tait. There may also be alterations, both local 
and general, in the consistency of the cytoplasm, which would 
favor the more extensive spreading-out of the cell. The consis- 
tency or viscosity of protoplasm is probably a variable char- 
acter, as shown by Chambers (717) and Seifriz (20). The mi- 
gration of the cells probably depends on alternating changes 
in the consistency of the protoplasm with changes in surface 
tension, as suggested by Loeb (20, ’21) for the amoeboid move- 
ments of the blood-cells of Limulus. 
In the many cultures examined we have never seen spread-out 
endothelial cells from the liver with long, slender, fine-branched 
processes such as are shown in figures 1 and 2 from subcutaneous 
tissue. Figures 7 and 8 show the slender more elongated type 
of cell, and figures 3, 4, 9, and 11, the more common type, which 
possesses a broad, thin, lateral expansion. Figures 5 and 6, 
a more unusual variety, show the cells forming a somewhat 
membranous or sheet-like outgrowth resembling the mesothe- 
lium-like membranes in cultures from the heart and intestine. 
They are different, however, and mesothelial membranes have 
not been observed in these liver cultures. 
