ORIGIN OF SPECIES 223 



lation of the spermatozoon to the ovum in fertiHza- 

 tion. 



Some cytologists (thus O. Hertwig) defined fer- 

 tilization as the fusion of the two nuclei, the nucleus 

 of the sperm and the nucleus of the egg. But this 

 definition had to be abandoned in the light of later 

 experimental knowledge. All the successful experi- 

 ments on artificial parthenogenesis disprove the idea 

 that the fusion of the two nuclei is necessary to 

 fertilize, or activate, the egg. In some experiments 

 by Boveri, "an enucleated fragment of an e^gg was 

 fertilized with a spermatozoon of a foreign species." 



It was Boveri's conclusion, shared by many later 

 authorities, that the centrosome of the spermatozoon 

 is the essential organ that brings about division in 

 the egg. However, it was found that this theory 

 claims too much for the centrosome as an organ. 



A flood of light has been shed upon the nature of 

 the process of fertilization by Jacques Loeb's remark- 

 able experiments on the artificial fertilization of 

 echinoderms — sea-urchin and starfish. 



Jacques Loeb has been quoted repeatedly; and 

 here it may be noted that Dr. Loeb's brilliant and 

 epoch-making work represents the highest point of 

 experimental knowledge of life thus far attained by 

 science. Needless to say, much work in this field of 

 artificial parthenogenesis and of experimental em- 



