34 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
greater for the egg of Strongylocentrotus purpuratus than for 
Arbacia. 
Sea-water is, according to van’t Hoff, a mixture of the 
following composition: 100 molecules NaCl, 2.2 molecules KCl, 
1.5 molecules CaCl,, 7.8 molecules MgCl., and 3.8 molecules 
MgSO,.' To this, traces of NaHCO; and Na,HPO, are to be 
added. The osmotic pressure of the ocean water shows local 
variations. The eggs of the animals in Pacific Grove develop 
best if the salts are used in a half grammolecular concentration. 
For the fauna in Woods Hole, on the Atlantic, a shghtly higher 
concentration, about 21/40 m is, perhaps, the optimum, al- 
though m/2 solutions give almost the same result. Solutions 
of 25/40 m are decidedly injurious. 
The reaction of the sea-water is slightly alkaline. In 
Pacific Grove the concentration of the free HO ions of the 
sea-water seems to lie between 10° and 10° N, since it is 
alkaline to neutral red but not to phenolphthalein. The 
concentration of the HO ions in the sea-water at Woods Hole 
is slightly higher and may reach 10° N. 
If we make m/2 solutions containing NaCl, KCl, CaCl, 
MgCl., and MgSO, in the right proportion, the newly fertilized 
eggs of Strongylocentrotus will, as a rule, not be able to develop 
to the larval stage in such a solution, unless the Cuo is above 
10-7 N. The eggs of various females differ slightly in their 
minimum CHo. It is, of course, necessary to free the eggs 
carefully from all traces of sea-water by washing them re- 
peatedly in neutral solutions before submitting them to the 
experiment. 
A van’t Hoff solution was prepared. The Co was about 
10° N, i.e., the solution was neutral. To 50 c.c. of this solution 
were added 0, 0.1, 0.2, 0.4, and 0.8 N/100 KOH. Newly 
fertilized eggs of Strongylocentrotus purpuratus were put into 
these solutions. In the neutral solution no egg developed 
1 We will call this solution for the sake of brevity the van’t Hoff solution. 
