Membrane Formation 



361 



some little distance. Two separate events take place, the forma- 

 tion of the membrane, and its separation from the egg. 



Mech 



ecnanism 



In order to simplify conditions as much as possible let us con- 

 sider what would occur under certain conditions in an egg cell 

 (Fig. 2) in which everythmg has been removed except those sub- 

 stances directly connected with the membrane reaction. Its 



Figs i and 2. 



inorganic analogue would be represented by an hypothetical cell 

 (Fig. i) containing cane sugar and appropriate enzymes. 



The tactions indicated above will proceed until all the sub- 

 stances are present in definite proportions. Equilibrium is then 

 attained. The egg (cell) membrane is impermeable to the con- 

 tained substances and also to the salts of sea-water. This repre- 

 sents the condition in the mature sea urchin egg. Suppose now 

 a momentary change of permeabihty occurs so that CO., + mem- 

 brane substance (COo + CoHjOH) may pass out of the system. 

 This upsets the equilibrium and the reactions proceed in the direc- 



