It is impossible to state at present whether the fertilization membrane is preformed in the fertilized egg and merely lifted off from the egg or whether its formation is due to the hardening of a colloidal substance separated from the emulsion (or excreted) and hardened in touch with sea water. But we can be sure of one thing, namely, that the liquid between egg and membrane contains some colloidal substance which determines the tension and spherical shape of the membrane. The membrane is obviously permeable not only to water but also to dissolved crystalloids, while it is impermeable to colloids. When we add some colloidal solution (_e. g._, white of egg, blood serum, or tannic acid) to the sea water containing fertilized eggs of _purpuratus_, the membrane collapses and lies close around the egg; while if the eggs are put back into sea water or a sugar solution the membrane soon assumes its spherical shape. This is intelligible on the assumption that in the process of membrane formation (or in the destruction of the emulsion in the cortical layer) a colloidal substance goes into solution which cannot diffuse into the sea water since the membrane is impermeable to the colloidal particles. The membrane is, however, permeable to the constituents of sea water or to sugar. Consequently sea water will diffuse into the space between membrane and egg until the tension of the membrane equals the osmotic pressure of the colloid dissolved in the space between egg and the membrane. If we add enough colloid to the outside solution so that its osmotic pressure is higher than that of the colloidal solution inside the membrane the latter will collapse. It should also be stated that the unfertilized eggs of many marine animals are surrounded by a jelly (chorion) which is dissolved when the egg is fertilized.[93] The writer has shown that the same chemical substances which will induce membrane formation and artificial parthenogenesis will as a rule also cause a swelling and liquefaction of the chorion. [93] It has been stated by several writers that the eggs of the sea urchin can no longer form the fertilization membrane when the jelly surrounding the egg is dissolved. The writer has found that if the jelly surrounding the eggs of _Strongylocentrotus purpuratus_ is dissolved by acid the eggs still form a fertilization membrane upon the entrance of a spermatozoön. We have devoted so much space to the mechanism of membrane formation since it is likely to give a clearer insight into the physicochemical nature of physiological processes than the phenomena of muscular stimulation and contraction or nerve stimulation, upon which the majority of physiologists base their conclusions concerning the mechanism of life phenomena.