34 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



greater for the egg of Strongylocentrotus purpuratus than for 

 Arbada. 



Sea-water is, according to van't Hoff, a mixture of the 

 following composition : 100 molecules NaCl, 2 . 2 molecules KC1, 

 1.5 molecules CaCl 2 , 7.8 molecules MgCl 2 , and 3.8 molecules 

 MgS0 4 . 1 To this, traces of NaHC0 3 and Na 2 HPO 4 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 slightly 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~ 6 and 10~ 5 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 maj^ reach 10~ 5 N. 



If we make m/2 solutions containing NaCl, KC1, CaCl 2 , 

 MgCl 2 , and MgS0 4 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 CHO is above 

 10~ 7 N. The eggs of various females differ slightly in their 

 minimum C HO. 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 CHO was about 

 10~ 7 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 



i We will call this solution for the sake of brevity the van't Hofif solution. 



