172 COMPARATIVE PHYSIOLOGY 



sea water. Further experiment showed that this was not a 

 specific effect of the magnesium ion at all, but could be re- 

 produced by increasing the osmotic pressure of the solution 

 with a number of different reagents. Exposure for two 

 hours to any one of the following mixtures suffices to induce 

 development up to the pluteus stage in eggs of Arbacia 

 when transferred back into normal sea water : — 



50 c.c. sea water, 50 c.c. 1*25 M, MgCIa ; 

 90 c.c. sea water, 10 c.c. 2*5 M, NaCl or KCl ; 

 100 c.c. sea water, 25 c.c. 2'o M, cane sugar ; 

 80 c.c. sea water, 17*5 c.c. 2*5 M, urea. 



M 



The freezing point of sea water is about of the order — 



to ^ NaCl. From inspection of the above it is clear that 

 8 



one salient feature is common to all these mixtures — they have 

 an osmotic pressure higher than that of sea water ; and since 

 the cell is in osmotic equilibrium with its environment, they 

 must tend to withdraw water from the egg. The following 

 table, taken from experiments of Loeb on another sea-urchin, 

 Strongylocentrotus purpuratus, indicates the optimum con- 

 centration and osmotic pressure of sea-water mixtures for 

 different reagents 



Less reliable results were obtained in subsequent experi- 

 ments on Strongylocentrotus than with Arbacia. And osmotic 

 activation alone did not produce a hundred per cent, yield 

 in either case. The plutei were in some respects abnormal 

 in that they did not swim near the surface ; there was a fairly 

 high mortality ; and — most significant of all — the eggs did 

 not form the characteristic investment, known as the fertilisa- 

 tion membrane which is an invariable consequence of normal 

 fertilisation by the agency of the sperm. This last fact 



