32 



FRANK R. LILLIE. 



The theory of the identity of fertiHzin and sperm agglutinating 

 substance rests upon a considerable body of ascertained facts (see 

 study VI), and it gives us at once a point of attack and a working 

 hypothesis of considerable value. I have been able to show for 

 instance that the origin of the capacity of the egg for being fer- 

 tilized can be understood on this basis; that the cessation of 

 fertilization capacity can also be so understood; and that the 

 physiological sterility (prevention of polyspermy) of fertilized 

 eggs is readily explained by the neutralization of the fertilizin 

 by a substance (anti-fertilizin) demonstrably present in the egg. 



On the other hand the theory does not postulate that the 

 fertilizin of all forms should agglutinate sperm of its own species. 

 There may be many forms in which the union of the sperm re- 

 ceptors with fertilizin does not produce such physical changes of 

 the spermatozoa as to lead to agglutination. In those cases in 

 which agglutination does occur we have a reaction very useful in 

 analysis; but it cannot be too strongly emphasized that the 

 agglutination itself is to be regarded merely as an indicator of 

 the essential reaction. 



