THE PERMEABILITY OF THE EGG MEMBRANE. 385 



After standing three and one-half hours the remainder of the 

 eggs were transferred to sea water and fertilized, but in no case 

 were any embryos formed. 



The above series of experiments goes to show that the term of 

 permeability of the egg membrane may be considerably pro- 

 longed by the exclusion of the calcium and magnesium salts. In 

 no case, however, were the eggs capable of fertilization after 

 standing for three and one half hours. It seems quite probable 

 that the membrane is affected by internal changes in the proto- 

 plasm, which must be very active at this period. It is also to be 

 noted that the ~yNa 2 SO 4 solution helps the egg to retain its 

 permeability longer than any of the ingredients of sea water. 

 Further experimentation might lead to the discovery of a medium 

 that would very greatly prolong the period of susceptibility to 

 fertilization. 



Conclusion. These experiments show clearly enough that 

 the membrane of the egg of Fitndnlus hctcroclitus is subject to 

 manipulation by the use of cations and anions, so that it becomes 

 permeable or impermeable to its own sperm at the will of the 

 .investigator. I am unable to formulate any explanation of these 

 changes in the condition of the membrane except that of a 

 reversible coagulation and the consequent reversible surface 

 tension. 



When the surface tension of the egg is increased by coagula- 

 tion of its colloids the sperm probably stands out on the surface 

 of this altered membrane much as a minute droplet of water 

 would stand out on the surface of an oil globule. Under ordi- 

 nary conditions, however, the sperm surface and the egg surface 

 are so adjusted to one another that they fuse on contact, and the 

 sperm substance is engulfed by that of the egg. 



We may suppose, then, that each species has a surface tension 

 for the egg that is exactly suited to that of its own sperm and 

 that, if altered, it becomes no longer suited to its own sperm but 

 may become suited to other closely related sperm. 



Loeb's results, cited at the beginning of this paper, offer evi- 

 dence in favor of this view. 



In the case of hermaphroditic animals, such as the tunicates 

 with which Morgan worked, the condition is undoubtedly differ- 



