588 STUDIES IN GENERAL PHYSIOLOGY 
It seems as tf the physical condition of the colloids was the 
essential point, and that this might be affected by various 
ton combinations in the same way. 
Solutions which allow the formation of a skeleton.—The 
next question was what ions should be added to the above- 
mentioned solutions in order to obtain plutei with a skeleton. 
A B 
oy 
ib 
on 
FIG. 147 
I found that a trace of Na,CO, kas that effect. In a solu- 
tion of 95 cc. §n NaCl + 2c.c. 12" CaCl, +1 cc. 2n KCl 
+ lec. $n NaCO, a skeleton was formed within three days. 
This skeleton was not quite normal. It showed a formation of 
knobs and spheres which I never saw in the skeleton formed 
in normal sea-water (Fig. 147B). I was anxious to obtain 
plutei with normal skeletons, and succeeded in doing so by 
adding a trace of MgCl, to the above-mentioned solution. 
The solution which yielded plutei with a normal skeleton con- 
sisted of 95 ec. $n NaCl +1 cc. 149n MgCl, +1 cc. En 
KCl + 2c. ¥9n CaCl, +1 cc. dn Na,CO,. The skele- 
ton is sketched in Fig. 147, A. It was therefore evident that 
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