2OO L. V. HEILBRUNN. 



initiation of development, either artificially produced or the 

 result of fertilization, always involves a gelatinization or coagu- 

 lation with the egg. This coagulative change is evoked before 

 the egg interior shows any other signs of the approach of develop- 

 ment. The coagulating substance (or substances) is evidently 

 very ready to coagulate. The ensemble of conditions within 

 the egg is no doubt responsible for this unstable state. Only 

 a slight change in salt concentration is then sufficient to bring 

 about coagulation. The fact that both increase and decrease 

 of salt concentration are effective, suggests that the protein 

 involved is of the globulin type. 1 



The mitotic spindle probably arises as a direct result of the 

 coagulative change. The actual explanation of how this occurs 

 is not truly a problem of biology but a problem of colloid chemis- 

 try, for it has been shown (Fischer, '99) that coagulation of 

 inanimate proteins can produce structures identical in appear- 

 ance with the mitotic spindle. 



IV. SUMMARY. 



1. The unfertilized Arbacia egg consists essentially of fluid 

 proteins, surrounded by a stiff membrane (the vitelline mem- 

 brane), which is the plasma membrane of the egg cell. This 

 membrane is a protein gel with little or no admixture of lipoids. 



2. There are two types of cortical change, membrane swelling 

 and membrane elevation. In the former, the membrane ab- 

 sorbs water and increases in thickness; in the latter, the normal 

 process, it becomes lifted away from the egg surface. 



3. The vitelline membrane only loses its semipermeable proper- 

 ties several minutes after elevation, and the increase of permea- 

 bility that then ensues is best regarded as a result rather than 

 as a cause of the process. 



4. Artificial membrane elevation is produced only by sub- 

 stances which lower the surface tension of the vitelline mem- 

 brane. This is explainable on the basis of a theory that considers 

 the various forces at play on the membrane. After its surface 

 tension is lowered, the forces exerted outward are stronger than 



1 If the protein referred to here is, as seems most likely, the substance which 

 forms the spindle fibers, the fact that it shows the properties of a globulin becomes 

 of significance. For globulins are the proteins most closely associated with con- 

 tractile processes, muscles consisting almost entirely of them. 



