108 Artificial Parthenogenesis 



ization 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 sepa- 

 rated 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 destruc- 

 tion 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 



