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IOWA ACADEMY OF SCIENCE Voi,. XXVII, 1920 
A. R. Moore ® finds that the resistance to hypertonic sea-water 
is least “immediately before and during each cytoplasmic division, 
and that the maximal resistance is shown 35 to 45 minutes after 
fertilization and just after each division.” More recently Lillie ^ 
(1916), has made an extensive study of the rhythmical changes 
in the resistance of the dividing sea-urchin egg to hypotonic sea- 
water, and has discussed the physiological significance of this 
rhythm. His experiments show clearly that at or about the time 
of formation of the cleavage furrow, a marked decline takes place 
in the resistance of the egg to hypotony, and cytolysis is then rapid 
and complete. After the cleavage furrow is fully formed the 
original resistance returns. A similar reversible decline of resist- 
ance takes place at the second and third cleavage, and is probably 
general for mitotic cell-division. The minimum of resistance is 
found during the formation of the furrow. Both the decline and 
the return of resistance are rapid, the greater part of each phase 
occupying four to five minutes. Some increase of susceptibility is 
apparent ten or twelve minutes before the first appearance of the 
furrow. Similar observations have been made by Herlant ® in the 
egg Paracentrotus lividus. 
From such experiments it appears that the resistance of the eggs 
to a variety of injurious agencies is least at the time when they 
are undergoing rapid change of form. To account for these 
rhythmical changes in the physiological state of the egg, Lillie ® 
(1909) puts forward the hypothesis that they are essentially the 
result of variations in the physical condition, especially the per- 
meability, of the surface-film or plasma-membrane, the latter 
undergoing a reversible increase in permeability at the time of 
cleavage. If a rythm of alternate increase and decrease of per- 
meability accompanies the rhythm of the mitotic process, it seems 
logical to infer that the entrance of solutes into the cell would 
occur most readily when there is a loss of semi-permeability. 
Accompanying this change would be a decrease of the electrical 
surface-polarization, and this in turn probably would alter the 
metabolic processes, especially oxidations within the cell. Cell 
metabolism then is inseparably bound up with cell-permeability; 
and the plasma-membrane, or semi-permeable surface-layer is 
something more than a haptogen membrane (to which it has 
frequently been compared). In discussing this subject in a later 
paper, Lillie makes it especially clear that this “general char- 
acteristic of semi-permeability (the all-essential insulating and 
diff using-preventing property) is not merely the result of a special 
