1/8 L. V. HE1LBRUNN. 



in one case at least, blood of the same individual from which 

 the eggs had been taken, was found to exert an accelerating 

 effect upon membrane swelling in "NaCl hypertonic sea-water." 

 If it be true that blood retards- acid swelling and favors salt 

 swelling, this fact can be used in cases of doubt, to determine 

 if a given type of membrane swelling is the result of the action 

 of a salt or an acid. 



F. Cortical Changes at Fertilization. 



The central object of studies in artificial parthenogensis is 

 to find an explanation of the processes occurring in normal ferti- 

 lization. The fact that artificial membrane elevation is apparent- 

 ly always the result of a lowered tension of the vitelline mem- 

 brane has of course led to the view that the spermatozoon also 

 produces a lowered surface tension. There are two conceivable 

 ways in which this could happen. In the first place the sperm 

 might carry a substance which lowers surface tension directly. 

 This is improbable, in view of the fact that it has not been pos- 

 sible to extract from sperm a membrane-elevating substance. 

 It is more logical to suppose that the very act of penetrating 

 on the part of the sperm lowers the tension. 



If the tension of a stretched thread be lowered at one point, 

 instantaneously the tension throughout the thread is lowered. 

 Similarly if the tension of a spherical stretched film or membrane 

 be lowered at one point, there will be a lowering of tension in 

 every point of the film, for in order that equilibrium be estab- 

 lished, the tension in every part of the spherical film must be 

 equal. This equalization of tension is probably a rapid process, 

 especially when not merely a point, but an appreciable area of 

 the surface has its tension lowered. 1 



Thus the penetration of the sperm almost immediately pro- 

 duces a lowered tension in all parts of the vitelline membrane. 

 In the sea-urchin egg, the sperm can not bore its way through 

 the membrane mechanically, as it is not provided with a per- 

 feratorium. It is therefore probable that the sperm has a 

 solvent action on the membrane upon coming in contact with 



1 However, in cases where a very thin film surrounds a large spherical mass as in 

 air-bubbles the attainment of equilibrium between the parts of the film is much 

 slower than the attainment of equilibrium between the film and the contiguous 

 fluids. Cf. Gibbs, 1. c., p. 300. 



