NATURAL DEATH AND FERTILIZATION 747 



the egg. Such an acceleration might, for example, be 

 brought about through certain ions (for example, the 

 hydrogen ions of nucleic acid), yet the possibility that 

 such catalytic effects might also be brought about through 

 enzymes or other substances is of course not shut out. Yet 

 this fact must be considered: that we have been able to 

 produce artificially normal embryos capable of development 

 through ions, while the careful experiments of Cries con- 

 ducted in my laboratory, in which he attempted to find the 

 same to hold for enzymes, have thus far failed. 



In conclusion I wish to thank my assistant, Mr. Neilson, 

 for the assistance which he rendered me in these experi- 

 ments. 



