132 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



It can be seen from this that solutions of various substances 

 which are all theoretically isosmotic do not on that account 

 possess the same degree of physiological activity. It is there- 

 fore necessary to differentiate between theoretically isosmotic 

 and physiologically isotonic solutions. The two values are 

 more nearly identical for the red blood corpuscles than for the 

 sea-urchin egg. This will be made clear by the following table 

 in which is set forth the optimum concentration of solutions of 

 various substances with regard to artificial parthenogenesis. For 

 this optimum concentration can be very sharply determined by 

 choosing a definite length of exposure, such as 55 minutes at 

 about 15 C. 



TABLE XVII 



OPTIMUM CONCENTRATION OF SOLUTIONS OF VARIOUS SUBSTANCES FOR 



ARTIFICIAL PARTHENOGENESIS 



Of course these values which were obtained with S. purpur- 

 atus at Pacific Grove do not apply as they stand to any given 

 form of sea-urchin in any given locality. We have no explana- 

 tion to offer for these discrepancies except in the case of sugars 

 where the discrepancy seems to find its explanation in the 

 abnormal physical behavior of concentrated solutions. It is 

 possible that these salts modify the permeability of the egg in 

 a different degree or sense and that this accounts for the dis- 

 crepancies between calculated and observed results. 



