1 68 L. V. HEILBRUXX. 



for membrane elevation, but such a solution of chloroform has 

 a surface tension against air just slightly below that of pure 

 water. 1 The great volatility of chloroform explains its inability 

 to lower markedly the tension at the water-air surface; it is 

 unable to accumulate in the surface film. But it can readily 

 accumulate at a solid-liquid surface and here it no doubt pro- 

 duces a marked lowering of surface tension. It follows that, 

 although by a measurement of the air-water surface tension we 

 can determine qualitatively whether a substance lowers the 

 surface tension of the membrane or not, any truly quantitative 

 measurements are impossible. 



In order to show qualitatively that a substance lowers the 

 surface tension of the membrane, it is only necessary to make 

 certain that its solution in water has a lower surface tension 

 than the pure solvent. The theoretical basis for this assertion 

 has already been pointed out. There is also another case to 

 be considered. Some substances exert a liquefying effect 

 upon the vitelline membrane. The liquefaction of a gel no 

 doubt results in a considerable lowering of its surface tension. 2 

 Ordinarily, however, the liquefying effect is a slow one, and 

 in such cases, membrane elevation does not occur. For if the 

 surface tension of the membrane is lowered only very slowly, 

 the egg proteins have time to follow the membrane as it is pulled 

 outwards. The result is an increase in egg diameter; the egg 

 cytolyzes. Thus when the surface tension of the vitelline 

 membrane is slowly lowered, cytolysis follows directly. 



On the other hand, when it is rapidly lowered membrane 

 elevation takes place first and cytolysis follows after a short 

 interval. After the vitelline membrane is elevated, it loses 

 its semipermeability and the hyaline layer takes its place as 

 the plasma-membrane of the egg cell (cf. p. 159). The same 

 forces act on this membrane as were found to act on the vitelline 

 membrane prior to its elevation. Hence when its surface tension 

 is lowered, since the osmotic pressure within the egg is no longer 

 completely counterbalanced, the hyaline layer tends to become 

 pushed outward. But the hyaline layer appears to be more 

 closely adherent to the egg proteins 3 so that they are pulled out 



1 Czapek ('n), p. 35. 



2 Cf. note p. 166. 



3 Probably because of a slight cortical coagulation. 



