Specificity in Fertilization 89 



6. The reason that an egg once fertilized with 

 sperm cannot be fertilized again may be found in a 

 group of facts which we will now discuss, namely, the 

 self-sterility of many hermaphrodites. The fact that 

 hermaphrodites are often self-sterile, while their eggs 

 can be fertilized with sperm from a different individual 

 of the same species has played a great r61e in the 

 theories of evolution. We are here only concerned 

 with the mechanism which determines the block to 

 the entrance of a spermatozoon into an egg of the 

 same hermaphroditic individual. 



Castle 1 observed and studied the phenomenon of 

 self -sterility in an Ascidian, Ciona intestinalis, which is 

 hermaphroditic. Animals which were kept isolated 

 discharged both eggs and sperm into the surrounding 

 sea water. Often no egg was fertilized, but in some 

 cases five, ten, or as many as fifty per cent, of the eggs 

 could be successfully fertilized with sperm from the 

 same individual; while if several individuals were put 

 into the same dish as a rule one hundred per cent, of 

 the eggs which were discharged segmented. Morgan 2 

 found that the eggs of various females differ in their 

 power of being fertilized by sperm of the same in- 

 dividual while one hundred per cent, could usually be 

 fertilized with sperm of a different individual. He 



1 Castle, W. E., Bull. Mus. Comp. Zool., Harvard, 1896, xxvii., 203. 



2 Morgan, T. H., Jour. Exper. Zool., 1904, i., 135; Arch.f. Entwcklngs- 

 tnech., 1910, xxx., 206. 



