602 STUDIES IN GENERAL PHYSIOLOGY 
for the Arbacia egg is shown by the fact that the fertilized 
eggs of Arbacia develop better in this solution than in 60c.c. 
20n MgCl, +40 c.c. sea-water. Hence it is evident that 
the Mg ions alone were not able to bring about the develop- 
ment of the unfertilized Arbacia eggs that had been in solu- 
tion 1. Either the presence of other ions, such as are 
contained in the 40 c.c. of sea-water or the increased osmotic 
pressure in the mixture of 60 c.c. %?n MgCl, + 40 c.c. sea- 
water is essential for the development. The osmotic pres- 
sure of a 1,2 MgCl, solution is not very different from that 
of sea-water. 
Second series.—It was, of course, my next task to repeat 
this experiment. I now knew which solution must be used 
in order to obtain parthenogenetic blastulz, but I wanted to 
find out how long the eggs must remain in this solution in 
order to develop into a blastula. I put a quantity of 
unfertilized eggs of one female into a solution of 60 c.c. 
29n MgCl, +40 c.c. sea-water at ten o’clock. At various 
intervals a portion of these eggs was taken out of the solu- 
tion and put back into 200 c.c. of normal sea-water. The 
first lot was put back into normal sea-water after thirty 
minutes. No egg had a membrane and none was seg- 
mented. The second lot was put back into normal sea-water 
after sixty-five minutes, the third lot after one hour and 
forty minutes, the fourth after one hour and fifty minutes, 
the fifth after two hours and five minutes, the sixth after 
two hours and fifteen minutes, lot 7 after two hours and 
thirty minutes, and the eighth lot after three hours and 
fifteen minutes. The first, second, and eighth lots differed 
much in regard to the time they were exposed to the artificial 
solution from lots 8, 4, 5, 6, and 7, which were taken out of 
the solution at much shorter intervals. No egg in any of 
these solutions had any membrane or showed any trace of 
segmentation at the time they were put back into normal 
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