THE PHYSIOLOGY OF FERTILIZATION 149 



become more permeable to certain intra-vitam dyes 

 immediately after fertilization. Harvey (1910) con- 

 firmed this and showed also a temporary increase in the 

 permeability toward alkalies. Lyon (1909) shows that 

 about double the amount of oxygen is liberated from 

 H 2 O 2 by fertilized eggs of sea urchins as compared with 

 equal amounts of unfertilized eggs, beginning about 

 three minutes after insemination. This may be ex- 

 plained by increased permeability by which peroxide 

 and catalase come more easily together. 



R. S. Lillie (1916-18) has shown that after fertili- . 

 zation Arbacia eggs take up water by osmosis several I 

 times more rapidly than before fertilization; if a mix- * 

 ture of fertilized and unfertilized eggs be placed in 

 hypotonic sea- water (forty parts sea-water plus sixty of 

 tap water) it is possible within two or three minutes to 

 distinguish the fertilized eggs by their greater diameter. 

 The volume of water entering the fertilized egg in three 

 minutes was found to be n Xio s ju 3 and the unfertilized 

 e gg> 3 6 X io s /x 3 . Osmotic equilibrium is reached within 

 about eight minutes in the case of the fertilized egg; 

 in the case of the unfertilized egg the entrance of water 

 is more gradual, and many of the eggs break down 

 before osmotic equilibrium is reached. Figure 18 fur- 

 nishes a comparison of the rate of entrance of water. 



The rate of entrance of water is essentially constant 

 during the period of exposure in the case of both ferti- 

 lized and unfertilized eggs relative to the existing gra- 

 dient of osmotic pressure, which demonstrates that the 

 differences between the two sorts is due to a difference 

 in the resistance of the membrane to the passage of 

 water. The amount of water which enters or leaves 



