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 to say. 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 2^ normal 

 NaCl or KC1 solution; that is, a solution which contains 

 about 186 g. of KC1 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- 



