MeEcHANISM OF MEMBRANE FORMATION 212 
but possessed some other shape. In such eggs the egg mem- 
brane at the start followed the contour of the egg. This proves 
that the primary factor in membrane formation is the swell- 
ing of a substance lying on the surface of the egg, similar to 
that which forms the chorion. The swollen substance then be- 
comes more and more liquid. Immediately after the forma- 
tion of the membrane it can be caused to collapse if we add a 
protein to the sea-water, which shows that in S. purpuratus 
only liquid matter lies between membrane and protoplasm. In 
some cases of artificial parthenogenesis, e.g., by bases or by 
fatty acids, in Arbacia we observe instead of a fertilization mem- 
brane a fine film surrounding the egg. It is possible that in 
this case the swelling is less complete and the membrane re- 
mains gelatinous. There may be all kinds of transition stages 
between the gelatinous film and the cases with a typical fer- 
tilization membrane separated from the protoplasm by a 
liquid. 
The existence of a colloid substance within the membrane 
chamber can be established by direct observation. For it can 
be seen that the fluid between the membrane and cytoplasm 
contains a constituent of somewhat higher refractive power 
than the sea-water. 
6. We have already mentioned that substances like ben- 
zol, saponin, ete., can cause both membrane formation and 
cytolysis. 
The first of the two is produced when they have time to 
affect only the surface of the egg; cytolysis is produced when 
their effect extends to the deeper layers of the egg. 
Now the cases of cytolysis afford a very pretty demonstra- 
tion of our theory. For since the greater the fraction of the 
egg which comes under the effect of the membrane-forming 
reagents, the greater the amount of colloid that must be lique- 
fied; hence a greater osmotic pressure should be set up by com- 
plete cytolysis than by simple membrane formation, and hence 
