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 Cheetop- 

 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 KC1 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. 2^n KC1, causes the 

 parthenogenetic development of the eggs of Chastopterus 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.c. sea-water + 2 

 c.c. 2^n KC1 or 97 c.c. sea-water + 3 c.c. 2-|w KC1 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 KC1, 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 -f- 10 c.c. 2^n KC1 sufficed to cause a great 

 many eggs to reach the blastula stage, a mixture of 90 c.c. 

 sea- water + 10 c.c. 2^n NaCl was practically ineffective. I 

 had to take 87^ c.c. sea-water + 12|^ c.c. 2-Jn NaCl. It is, 

 however, possible, that this difference is only apparent. As 

 the sea-water consists chiefly of NaCl, the addition of 10 c.c. 

 of a 2^n NaCl to 90 c.c. sea- water will increase the osmotic 

 pressure of the sea-water less than the addition of 10 c.c. of 

 a 2^n KC1 solution, as the degree of dissociation is less if 

 the concentration is higher. Further experiments with pure 

 NaCl and KC1 solutions will have to decide whether the dif- 



