680 STUDIES IN GENERAL PHYSIOLOGY 
do not need to stay so long in the more concentrated solution 
as the eggs of Arbacia. 
Although in this regard the difference between Chetop- 
terus and Arbacia is slight, a very striking difference exists 
in regard to the specific effects of K ions upon the develop- 
ment. While a pure KCl solution of lower osmotic pressure 
than sea-water, or sea-water with a slight increase of K, e.g., 
a mixture of 98 c.c. sea-water + 2 c.c. 24” KCl, causes the 
parthenogenetic development of the eggs of Cheetopterus that 
have been exposed to such a solution only a few minutes, 
such solutions are without any effect upon the unfertilized 
eggs of sea-urchins (Arbacia). I left the unfertilized eggs 
of Arbacia repeatedly in a mixture of 98 ¢.c. sea-water + 2 
c.c. 24n KCl or 97 ¢.c. sea-water + 3 ¢.c. 24n KCl for from 
three minutes to twenty-four hours without any develop- 
ment following, with the exception of a few eggs that reached 
the two-cell stage after about twenty hours. But this hap- 
pens just as well in normal sea-water. 
As far as the Arbacia eggs are concerned, I can only state 
that if we increase the osmotic pressure of the sea-water by 
adding KCI, a slightly smaller increase in the osmotic pres- 
sure is required to bring about the parthenogenetic develop- 
ment than if we add NaCl. I found regularly that while 
90 c.c. sea-water + 10 c.c. 24n KCl sufficed to cause a great 
many eggs to reach the blastula stage, a mixture of 90 c.c. 
sea-water + 10 c.c. 24m NaCl was practically ineffective. I 
had to take 874 c.c. sea-water +124 c.c. 2in NaCl. It is, 
however, possible, that this difference is only apparent. As 
the sea-water consists chiefly of NaCl, the addition of 10 cc. 
of a 24 NaCl to 90 ¢.c. sea-water will increase the osmotic 
pressure of the sea-water less than the addition of 10 e.c. of 
a 24n KCl solution, as the degree of dissociation is less if 
the concentration is higher. Further experiments with pure 
NaCl and KCl solutions will have to decide whether the dif- 
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