STUDIES IN ARTIFICIAL PARTHENOGENESI-. 179 



it. The partial liquefaction or swelling of the vitelline mem- 

 brane at the point of sperm entrance can be conceived of as 

 serving two functions: (i) it enables the sperm to enter, (2) it 

 lowers the surface tension of the membrane and thus produces 

 membrane elevation. 1 



That the sperm actually does produce a substance capable 

 of causing membrane swelling, can be demonstrated. It is not 

 possible to observe the swelling produced by a single sperm. 

 But if the eggs are placed into very concentrated sperm suspen- 

 sions, the vitelline membrane can be seen to swell all around 

 the egg. Such concentrated suspensions are obtained by al- 

 lowing the sea-urchins to shed their sperm. As is well known, 

 the shedding reaction is aroused when the oral part of the shell 

 is cut away. If the "dry" sperm be diluted only very slightly, 

 an enormous sperm concentration can be obtained. When 

 eggs are mixed with sperm suspensions of such high concentra- 

 tion, each egg immediately becomes surrounded by a halo of 

 wriggling sperm. Soon the vitelline membrane can be seen 

 slowly to increase in thickness, it swells until it may reach a 

 thickness of about 3 microns. That normal membrane elevation 

 has not taken place can be shown by the fact that the swollen 

 membranes thus produced do not collapse when the eggs are 

 placed in a i per cent, or a 2 per cent, albumen solution (in sea- 

 water). In a concentrated' sperm suspension, each point on 

 the vitelline membrane is a point of attack on the part of the 

 spermatozoa, and the entire membrane becomes swollen. 2 



The concentration of sperm necessary to produce a complete 



1 It might be thought that puncture of the vitelline membrane, e. g., by a needle, 

 should produce elevation. But this is not necessarily the case, for the hole pro- 

 duced by a mechanical puncture of the membrane, if not immediately closed, 

 would involve a loss of its semipermeable properties, and these on the basis of the 

 theory (see pp. 165-166) are necessary for membrane elevation. A deep prick 

 would also produce coagulation of the underlying cytoplasm, which would tend to 

 prevent elevation. 



2 It might be asked why elevation of the membrane does not follow swelling 

 produced by concentrated sperm suspensions, since this no doubt results in a rapid 

 lowering of surface tension. The answer is clear. In a previous paper (Heilbrunn, 

 '13) it was pointed out that membrane elevation never occurred when the egg or 

 its cortex was coagulated. Now in concentrated sperm suspensions, it can be 

 shown that a profound coagulation does take place. This was demonstrated by 

 a method which has been developed for revealing the presence of coagulation 

 within the egg. For details of this method see p. 192. 



