170 PROBLEMS OF FERTILIZATION 



VI. EXTERNAL CONDITIONS OF FERTILIZATION 



The fertilization reactions that we have been con- 

 sidering are dependent, not only on the internal condi- 

 tions of the gametes, but also on the nature of the 

 medium in which insemination occurs. Fertilization 

 always occurs in an aqueous medium containing a bal- 

 anced solution of salts, of which NaCl, MgCl 2 , KC1, and 

 CaCl 2 are the chief. In such a medium fertilization 

 depends, within the usual range of temperature, on re- 

 action of the medium (acidity or alkalinity) and the 

 balance and concentration of the salts. Sea-water is a 

 medium of this kind in which the variable factors can 

 be readily controlled. Most studies have therefore been 

 made on marine animals; but there is abundant evidence 

 that the same principles apply to other animals. 



The effect of reaction of the medium may be shown 

 by some hitherto unpublished experiments on starfish 

 eggs made in 1914. In the summer these eggs (at 

 Woods Hole) frequently do not fertilize very readily 

 in normal sea-water, but they may be made to do so 

 by a slight increase in the alkalinity; on the other 

 hand an increase of acidity tends to inhibit fertilization ; 

 the following experiment demonstrates the principle. A 

 series of eleven watch glasses is laid out in the order i 

 to n, of which the center one (No. 6) contains normal 

 sea-water, those to the left increasing concentrations of 

 HC1, and those to the right increasing concentra- 

 tions of NaOH in sea- water as follows: No. i, N/$oo 

 HC1; No. 2, N/ 1,000 HC1; No. 3, N/ '2,000 HC1; No. 4, 

 N/ 4,000 HC1; No. 5, N/ 10,000 HC1; No. 6, normal sea- 

 water; No. 7, #/ 10,000 NaOH; No. 8, N/4,ooo NaOH; 

 No. 9, N/2,000 NaOH; No. 10, N/i,ooo NaOH; No. n, 



