EXPERIMENTS ON ARTIFICIAL PARTHENOGENESIS 767 
all my experience that the addition of potassium or sodium 
salts, especially potassium chloride and sodium chloride, is 
perhaps the best. The degree of increase in concentration 
is of great importance. If the correct concentration is not 
struck, failure will result, and it is remarkable how greatly 
the necessary concentration varies in different series of 
experiments. Whether the variations are exclusively of an 
individual character and correspond to the different states of 
maturation of the eggs I will not endeavor tosay. Possibly 
temperature also has some effect. In order to meet all these 
possibilities I always work with a series of solutions. In 
this way I am certain to obtain good results in at least one 
of the solutions. I use as a stock solution a 24 normal 
NaCl or KCl solution; that is, a solution which contains 
about 186 g. of KCl in the liter of solution. In my experi- 
ments the solutions were accurately titrated, but this is 
superfluous for most purposes. I take six dishes, each con- 
taining 100 c.c. sea-water, and add to these six dishes the 
series of 8, 10, 12, 14, 16, 18 c.c. of the above 2$ normal 
NaCl solution. The unfertilized eggs of the sea-urchin are 
then distributed into these six dishes (and in addition into a 
control dish containing pure sea-water). 
Differences also exist regarding the time during which 
the eggs must remain in these solutions. It is therefore 
necessary to remove, not all the eggs at once, but at various 
intervals after about one-half, one, one and one-half, and two 
hours. In this way one will certainly strike the optimal 
concentration and time of experiment. Potassium chloride 
has the advantage that it leads usually to the formation of a 
single embryo from each egg, while when sodium chloride 
is used more than one embryo is usually formed from an egg. 
The formation of the skeleton, however, probably occurs 
somewhat better when sodium salts are employed than when 
potassium salts are used. Mr. Hunter obtained very satis- 
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