640 STUDIES IN GENERAL PHYSIOLOGY 
KCl solution only a comparatively small number of eggs 
reached the blastula stage, certainly many less than in my 
previous experiments with MgCl, on Arbacia. A further 
examination revealed the fact that the MgCl, solution 
which I had used was, through an error or a misunder- 
standing of the assistant who made it, weaker than a 22n 
solution. As soon as I found this out, I started experi- 
ments with more diluted NaCl and KCl solution. Instead 
of using equal parts of a 142n NaCl or’ KCl solution and 
sea-water, I used the following mixtures: 
20 42n NaCl-+ 30 distilled water + 50 sea-water, 
Or— —- 174 42. NaCl + 323 distilled water + 50 sea-water. 
In both cases more eggs reached the blastula and pluteus 
stage than with the original stronger mixture. In one case 
unfertilized eggs developed beautifully after having been 
for two hours in a solution of equal parts of 15 21n NaCl 
+ 35 distilled water-+50 sea-water. But this was nearly 
the lowest limit for artificial parthenogenesis in Arbacia. 
Asa rule, 25 per cent. or more of the unfertilized Arbacia 
eggs reached the blastula stage. 
3. It was thus proved that MgCl, does not play a specific 
role in the production of artificial parthenogenesis. It 
remained to decide whether it is essential to remove one 
part of the normal constituents of the sea-water, or whether 
the mere increase of the osmotic pressure suffices. I found 
that the increase in the osmotic pressure of the sea-water is 
all that is needed. In the experiments in which the maximal 
number of unfertilized eggs reached the bastula stage about 
1 gram NaCl had been added to the sea-water. We can 
produce the same increase in the osmotic pressure of the 
sea-water by adding 10 c.c. of the 24n NaCl or 24n KCl 
solution* to 90 ¢.c. of sea-water. In this case the mixture 
1My 24%4nNaCl solution contained 146.25 g. in a liter. The 2%nKCl solution 
contained 186.25 g. in a liter. 
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