HISTORY OF THE FERTILIZATION PROBLEM 23 



inferred from the necessary, from the inhibiting, and 

 from the favoring conditions of the medium. 



The fertilization reaction, like all biological reactions, 

 requires certain conditions of the environment, such 

 as definite range of temperature and chemical compo- 

 sition of the medium. In the first place, if these are 

 exceeded in either direction so far as to injure the cells 

 the fertilization reaction either does not take place or it 

 is rendered abnormal. The cause of the failure, or the 

 abnormality, in such cases lies in some change of the 

 internal composition of one or the other of the germ 

 cells. The classic experiments of this kind are those 

 of Oskar and Richard Her twig published in 1887. 

 These 'investigators studied the effects of high tempera- 

 ture, of various injurious chemical reagents, and of 

 mechanical shock on the germ cells separately before 

 fertilization, and on the process of fertilization itself 

 at various stages. Many exceedingly interesting obser- 

 vations were made, and problems were raised that were 

 not then ripe for solution. Other experiments of a 

 similar kind have since been made, but their considera- 

 tion properly belongs to the problems of the internal 

 factors, for the phenomena observed depend upon inter- 

 nal changes of the germ cells. 



In the second place, there may be modifications of 

 the medium which do not directly injure the germ cells, 

 but which inhibit or favor the fertilization reaction. 

 Examples of inhibiting phenomena are found in Pro- 

 fessor Loeb's studies of the relations of ions to the ferti- 

 lization reaction, or my own on the inhibiting action 

 of blood or tissue secretions of the same species on 

 fertilization. The most striking example of conditions 



