60 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
This is due to the fact that MgCl, and CaCl, dissociate into 
three ions. In sea-water, however, there are not 12 c.c. of 
MgCl, but 7.8 ¢.c. MgCl, +3.8c¢.c. MgSO,, 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 NaC] solution is small. 
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. of sea-water+10 c.e. 
of 24 m NaCl or KCl. 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+25 ¢.c. 2m cane sugar. 
80 c.c. of sea-water+173 c.c. 23 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+40 c.c. distilled 
water or even in 55c.c. 2m cane sugar+45 c.c. distilled water.* 
Tn the last case there is only a very slight increase of molecular 
concentration.* 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°. 
2Am. Jour. Physiol., IV, 178, 1900. 3 Tbid. 
4 We shall see later that the osmotic effect of a pure cane-sugar solution is. 
considerably higher than its theoretically calculated value. 
