60 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



This is due to the fact that MgCl 2 and CaCl 2 dissociate into 

 three ions. In sea-water, however, there are not 12 c.c. of 

 MgCl 2 , but 7. 8 c.c. MgCl 2 +3.8c.c. MgSO 4 , and hence a really 

 m/2 van't Hoff solution has a little weaker osmotic pressure. 

 At the same time, the difference in osmotic pressure between 

 a van't Hoff solution and a m/2 NaCl solution is small. 1 



Now I found that in Woods Hole artificial parthenogenesis 

 may be produced in sea-urchin eggs if they are left for between 

 one and two hours in a mixture of 90 c.c. of sea-water +10 c.c. 

 of 2^ m NaCl or KC1. The increase of osmotic pressure of this 

 solution was relatively small, amounting to about 40 per cent 

 of the osmotic pressure of the sea-water. 2 I also obtained 

 swimming larvae when the eggs of Arbacia were put for about 

 two hours in the following solutions: 



100 c.c. of sea : water -f- 25 c.c. 2 m cane sugar. 

 80 c.c. of sea-water+17| c.c. 2| m urea. 



In this case also only a very slight increase of molecular con- 

 centration takes place. 



A pure cane-sugar solution of relatively low molecular con- 

 centration is alone sufficient to cause unfertilized eggs of sea- 

 urchins to develop. Thus eggs of Arbacia were placed for two 

 hours in a solution of 60 c.c. 2 m cane sugar -f-40 c.c. distilled 

 water or even in 55 c.c. 2m cane sugar+45 c.c. distilled water. 3 

 In the last case there is only a very slight increase of molecular 

 concentration. 4 Nevertheless this treatment activates the egg, 

 though its development never goes farther than the blastula 

 stage, and, as a rule, not even so far. The development of 

 the egg generally ceases in the early segmentation stages. We 

 shall see later that the production of plutei is favored if the 



1 According to Garrey, the sea-water at Pacific Grove lowers the freezing- 

 point by 1 . 90. 



2 Am. Jour. Physiol., IV, 178, 1900. 3 Ibid. 



* We shall see later that the osmotic effect of a pure cane-sugar solution is 

 considerably higher than its theoretically calculated value. 



