THE PERMEABILITY OF THE EGG MEMBRANE. 379 



Among the possible explanations of this phenomenon Morgan 

 suggests "that the surface tension of the egg is of such a sort 

 that it excludes the sperm of the same individual, but this idea 

 does not appear to give a satisfactory solution, for, aside from the 

 fact that it is difficult to imagine how such a relation could exist, 

 there would also occur cases in which the surface tension of the 

 eggs of other individuals would exclude certain sperm, and this 

 would not appear to be the case. It is true that the addition of 

 the ether to the water may cause a difference in the surface ten- 

 sion of the egg, and it might be made to appear that this was the 

 way in which the self-fertilization is effected in ether solutions, 

 but I cannot believe that this is the explanation, because other 

 experiments show that a considerable amount of ether is neces- 

 sary to cause self-fertilization." 



In a later paper, 1 dealing with the same tunicates, Morgan cor- 

 rects his previous observations to some extent and shows by a 

 series of experiments that " it is not true that the sperm of a given 

 individual will fertilize equally well the eggs of all other indi- 

 viduals." 



As a matter of fact his tables show that the sperm of some 

 individuals is wholly ineffective on the eggs of other individuals, 

 and that all degrees of effectiveness are found. Thus his later 

 experiments serve to offset his chief objection (quoted above) to 

 the idea that surface tension might be an effective factor in con- 

 trolling the entrance of the sperm into the egg. 



Morgan showed that the addition of certain amounts of ether 

 to sea water made the sperm more active, and he inferred from 

 this fact that the activity of the sperm was the factor governing 

 fertilization. On the other hand, he states that eggs have been 

 fertilized by very inactive sperm. This seems to indicate that 

 the activity of the sperm is a subordinate factor in fertilization. 

 Moreover, no matter how slow-moving the sperm might be, 

 some would undoubtedly be able to reach some eggs. The 

 failure to fertilize would not then be due, I believe, to a failure of 

 the sperm to reach the egg-membrane, but rather to a failure to 

 fuse with and penetrate the latter. The fusion, as both Loeb 

 and Morgan have suggested, may very readily be dependent 

 upon surface tension. 



1 Morgan, BIOLOGICAL BULLETIN, Vol. VIII., No. 6, 1905. 



