THE PHYSIOLOGY OF CELL-DIVISION 699 
calcium to the solution. It is therefore to be expected that the 
initiation of cleavage by such solutions, if dependent on rapid 
increase of permeability, will also be prevented by the addition 
of calcium. This has been found to be the case. 
EXPERIMENTAL 
In the following experiments unfertilized eggs of Arbacia 
were exposed for five or ten minutes to isotonic solutions of neutral 
salts of sodium and potassium. The effects produced by pure 
solution and by solutions containing calcium chloride were 
studied and compared. The salts used were chlorate, nitrate, 
iodide, and sulphocyanate.’ The effect of after-treatment with 
hypertonic sea-water and cyanide-containing sea-water was also 
studied. 
The procedure was the same as in my former experiments. 
In each series eggs were exposed for five and ten minutes to (a) 
the pure solution of the salt and (b) to the same solution plus 
a definite quantity of “ CaCl?: usually 12.5, 15, or 25 ec. $ CaCl 
was added to 250 ce. of the pure salt solution. The concentra- 
tion of the latter was 0.55 M. 
The essential results of these experiments may be described 
in a few words. Eggs exposed for five or ten minutes to pure 
isotonic sodium or potassium iodide or sulphocyanate solutions 
and then returned to sea-water undergo the series of changes 
described in my former paper. <A large proportion, in favorable 
experiments practically all, form fertilization membranes, usually 
thin and close to the egg-surface; two or three hours later the eggs 
are found for the most part to be more or less irregular or amoeboid 
in form and in some cases subdivided into several cells. These 
cleavages are almost always irregular and much slower than nor- 
7 Chloride and bromide were omitted as practically inactive. The salts, it will 
be noted, are all salts of strong monobasic acids; the solutions were thus neutral 
in reaction in all cases. Since traces of alkali may produce membranes it is im- 
portant to make sure of this. Commercial sulphocyanate preparations are often 
distinctly alkaline; those used in the present experiments were neutral to phenol- 
phthalein. The OH-ion concentration was thus below 108 n—less than that of 
the Woods Hole sea-water. The salts used were Kahlbaum’s and Baker’s 
analysed. 
