STUDIES IN ARTIFICIAL PARTHENOGENESIS. 167 



in contact with the egg cytoplasm. It is, accordingly, now ex- 

 posed on both of its faces to a solution which has the power of 

 coagulating it. It might be expected that the process of gelatini- 

 zation or coagulation be carried a step farther, and that the 

 membrane become more rigid. This increased coagulation, 

 if I may so speak of the process, 1 results, as is typical for such 

 cases, in an increase of refractive index, and the optical image 

 of the membrane becomes more clearly defined. It may also 

 result in an increased permeability of the membrane, for Quincke 

 ('77) claims that precipitation membranes lose their semiper- 

 meable properties on becoming rigid. 



On the basis of the above theory, it is evident that when the 

 surface tension of the membrane drops below a certain limiting 

 value, elevation -occurs. The determination of this value is 

 at present impossible. It might be thought that the determi- 

 nation of the surface tension of the various membrane-elevating 

 solutions at their surface of contact with air, would throw some 

 light upon the matter. 2 But the lowering effect of a substance 

 on a solid-liquid (or liquid-liquid) surface tension can not be 

 measured by the effect the same substace produces on a gas- 

 liquid surface. The chief reasons for this are: (i) Adsorption 

 of added substances plays a very decided role in the case of 

 the solid-liquid surface, different solids of course showing dif- 

 ferent degrees of selective adsorption, (2) A solid-liquid surface 

 is not subject to the action of evaporation, as is a gas-liquid 

 surface. This is of especial importance if the substance which 

 lowers surface tension is volatile. In discussing films like 

 those of bubbles, Gibbs 3 says: "But when a component which 

 greatly diminishes the tension of the film although forming but 

 a small fraction of its mass (therefore existing in excess at the 

 surface), is volatile, the effect of evaporation and condensation 

 may be considerable, even when the mean value of the potential 

 for that component is the same in the film as in the surrounding 

 atmosphere." 4 Thus chloroform in water is quite effective 



1 It might also be designated as "loss of water." 



2 Czapek ('n) has thus endeavored to draw conclusions as to the surface tension 

 of the plasma membrane of plant cells by measurements of air-liquid surface tensions. 



3 L. c., p. 310. 



4 A condition which would at least be approximated if the film were in equilibrium 

 with its vapor in an enclosed chamber. 



