MeEcHANISM OF MEMBRANE FORMATION 209 
is permeable to salts and water but not for colloids. Hence 
the addition of a colloid te 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.’ 
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 smal! drops. Now it appears to 
me (so far as the osmotic properties of the membrane are 
1 Loeb, op. cit. 
