LARVAE OF ARBACIA PUNCTULATA 423 



and 10-5 N, while that of Woods Hole is somewhat higher and 

 may reach 10 -^ N." Moore, Roaf and Whitely give that of 

 Port Erin Bay, using the same indicator, phenol-phthalein, as 

 0.00023N, or 2.3 X 10-^ N. 



The degree of alkalinity necessary for development, moreover, 

 depends upon the character of the salts present. Herbst found 

 that the most favorable alkalinity of his artificial sea-water 

 was greater than that of the natural sea-water, yet addition of 

 hydroxyl to the latter caused no acceleration of growth* He 

 offered as a suggestion that possibly the concentration of hy- 

 droxyl could be less in the presence of such salts as the phos- 

 phates and carbonates of the sea-water, or else the phenol- 

 phthalein is altered in its reaction in the presence, of some 

 salts found in the sea-water but not in his artificial solution. 

 These facts indicate that the alkalinity of the sea- water is not a 

 constant factor, and that, even under the same degree of hy- 

 droxyl concentration, the effects vary with varying concen- 

 trations of salts. That the latter are not always present in 

 different waters in constant amounts has been adequately de- 

 termined. Mayer ('14) states that a 0.60M NaCl solution is 

 isotonic with the sea-water at Tortugas. Garrey ('04) gives 

 0.52M as equivalent to that at Woods Hole, 0.54M for Pacific 

 Grove and 0.58M for Beaufort. 



In the above experiments I have attempted to show some of 

 the modifications produced by these factors operating separately. 

 To determine the effects of all possible combinations , and of 

 gradual subjection would require much additional investigation. 

 But since it is impossible to alter one constituent of the sea-water 

 without involving other changes, some evidence of the com- 

 bined effects of several independent factors has been produced; 

 for instance, in the employment of 0.60M NaCl solutions dilu- 

 tion of the other constituents occurs coincident with increase 

 of osmotic pressure; in the carbonate solutions decrease in the 

 CaCOs— and MgCOs— content occurs at the same time as 

 increase of alkalinity, and, in the gradual concentration of 

 the sea-Water, a condition of increased density replaces one of 

 highly decreased concentration. 



