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 CaClj, 7.8 molecules MgClj, and 3.8 molecules 

 MgS04.' To this, traces of NaHCOg and Na2HP04 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"^ 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, CaCl2, 

 MgCl2, and MgS04 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"^ 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 Cho 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 



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



