172 T. H. Morgan. 



observed nothing that would support such a conclusion, and this 

 interpretation of the process would be foreign to what we find 

 in general in connection with fertilization even in cases where the 

 sperm of one species does not fertilize the eggs of another. 



We come now to a more subtile argument, and one that we 

 are scarcely in position to discuss profitably in our present state 

 of ignorance concerning the union of egg and spermatozoon. 

 It may be assumed that there is some sort of "chemical affinity" 

 between the egg and the spermatozoon that causes the two to 

 unite when they come together. On this assumption we should 

 have to suppose in Ciona that this affinity does not exist, or at 

 least is less strong, between the egg and the spermatozoa of the 

 same individual than between those of different individuals. Such 

 a statement carries us no further, however, than the facts, and in 

 the case of Cynthia we should have to assume that the affinity is 

 so nicely balanced that sometimes the spermatozoon can unite, and 

 sometimes it can not. In the case of Molgula the affinity must 

 be assumed to suffice to bring about self-fertilization. Until we 

 can give some more tangible form to this idea it does not appear 

 to have any greater value, than the mere statement of the facts, 

 and indeed may have less value, since it may give a wrong im- 

 pression as to the real factors at work. 



Finally there might be advanced what may be called the electro- 

 chemical hypothesis. The union of the egg and the spermatozoon 

 may be supposed to be an electrical phenomenon, connected with 

 a difference in the chemical composition of the two elements. The 

 sperm head is almost pure nuclear chromatin, while the surface 

 of the egg is protoplasmic. Possibly the spermatozoon and the 

 egg have different electrical charges and unite with each other 

 if brought near enough for the charges to become effective. But 

 on this supposition it is not clear why the eggs and the sperm of 

 the same individual would not unite. Here also we get no light 

 on the absence of self-fertilization in Ciona. 



I have kept constantly in mind while at work on this problem 

 the possibility that the spermatozoon may really enter the egg, but 

 fail to develop there, or fail to start the development of the egg, 

 because, coming from the same individual, it was not sufficiently 



