MECHANISM OF MEMBRANE FORMATION 209 



is permeable to salts and water but not for colloids. Hence 

 the addition of a colloid to the external solution increases the 

 osmotic pressure of the latter (although only slightly), but this 

 difference is enough to cause all the sea-water contained between 

 fertilization membrane and protoplasm to diffuse out, and as 

 a consequence the membrane collapses as shown in Figs. 61 

 and 62. If restored to normal sea-water without colloids, the 

 membrane becomes spherical again. These experiments suc- 

 ceed immediately after the membrane is formed. 



3. These data give us the key to the conception of the mech- 

 anism of membrane formation. The fertilization membrane is 

 perfectly spherical in form. This implies that the membrane 

 is in the condition of tension. Since, then, the experiments 

 already described prove that in Strongylocentrotus the fertiliza- 

 tion membrane is easily permeable to sea-water, there must 

 prevail within the membrane an osmotic pressure which is 

 equal to the tension of the membrane. This pressure must be 

 due to a substance which is contained within the membrane 

 cavity and cannot diffuse out through it. This substance must 

 be a colloid. The existence of such a colloidal, indiffusable 

 substance within the membrane cavity also explains the above- 

 mentioned fact that when the membrane has been caused to 

 collapse by the addition of serum to the sea-water, it can be 

 restored to its normal condition of tension by replacing the egg 

 in ordinary sea-water. Another fact that is also explained by 

 this hypothesis is that the membrane does not begin to collapse 

 until a certain definite mass of serum or protein has been added 

 to the sea-water. 1 



We mentioned in the first chapter that when the membrane 

 is slowly produced it can be observed that this process starts 

 in a roughening of the surface of the egg, and that this is due 

 to the formation of numerous small drops. Now it appears to 

 me (so far as the osmotic properties of the membrane are 



1 Loeb, op. cit. 



