THE PHYSIOLOGY OF CELL-DIVISION 697 
the ions act primarily on the plasma-membrane, altering the 
permeability of the latter, with corresponding changes in the 
electrical and other physiological properties of the tissue.* The 
influence of salts in initiating cell-division is, on the present 
theory, an instance of an essentially similar kind. 
Since in Arbacia eggs only the most active salts, iodide and 
sulphocyanate, and to a less degree nitrate, have decided action 
in inducing cleavage, while all increase the surface-permeability, 
though at different rates, it may be inferred that the rate of 
permeability-increase (@?) must exceed a certain critical mini- 
mum in order that cleavage may be started; hence the more 
slowly acting salts—chloride, bromide, and acetate—are prac- 
tically ineffective with these eggs. In Asterias eggs, on the other 
hand, the permeability of the plasma-membrane is more readily 
increased, and all of the above salts‘ produce membranes and start 
cleavage. Different eggs thus vary in their response to a given 
method of treatment according to the specific peculiarities of 
their plasma-membranes. In Asterias the plasma-membranes 
are evidently more susceptible to changes of permeability than 
in Arbacia. The former eggs are less stable, less uniform in 
behavior, and less tenacious of life; and these peculiarities are 
undoubtedly related to the greater ease with which they may be 
induced by artificial means to cleave and develop. It is self- 
evident that plasma-membranes which are highly impermeable 
and whose permeability is increased with difficulty must isolate 
the protoplasmic complex more completely from its surroundings 
than membranes of a more permeable and less stable kind, and 
that a more radical change in external conditions will be required 
to influence the metabolic or other processes in eggs thus enclosed. 
It is, I believe, for this reason that Arbacia eggs are relatively 
resistant and unresponsive as compared with Asterias eggs. 
Analogous considerations apply in other cases; hence the prob- 
3 Cf. Héber: Physikalische Chemie der Zelle und der Gewebe. Engelmann, 
Leipzig, 1906, pp. 217 seq., 370 seq. Further references in this book. 
4 Possibly excepting acetate whose action on starfish eggs I have not yet tried. 
I am confident, however, from the effects produced by chloride, that acetate will 
be found to have well-marked action on Asterias eggs. 
