26 PROBLEMS OF FERTILIZATION 



appears to be irreversible ; and the appearance of reversal 

 in parthenogenesis may be referred, like superposition 

 of fertilization on parthenogenesis, to incompleteness of 

 the initial reaction. 



3. Specificity is an outstanding feature of the ferti- 

 lization reaction, the significance of which is not weak- 

 ened by any hybridization experiments. We need not 

 stop to define the limits or the consequences of hybridi- 

 zation in order to justify the assertion that no theory of 

 fertilization which fails to include the factor of specificity 

 as one of the prime elements can be true. 



The fundamental character of specificity is illumi- 

 nated by the phenomena of self-sterility; in species 

 where this occurs the eggs and sperm of the same indi- 

 vidual are sterile inter se, though fertile with those of 

 all other individuals. This has led some botanists to 

 the conception of individual stuffs; but Correns' ex- 

 perimental analysis led him to the conclusion that the 

 specific factor is not an individual stuff but a definite 

 combination of stuffs for each individual. The combi- 

 nation arises always with the individual and disappears 

 with it. The only biological parallel of such phenomena 

 is found in the individual blood composition revealed by 

 serological studies. That there is a common factor in 

 species and individual specificity no one who has studied 

 both sets of phenomena can doubt. 



A consistent theory of fertilization must take account 

 of all these phenomena, not only the internal factors of 

 maturity of germ cells and the specificity of their reac- 

 tions, but also the external factors that favor or inhibit 

 the reaction. I have attempted to show in a series of 

 papers that the fertilizable condition of the egg depends 



