ORIGIN OF SPECIES 219 



tion. (See p. 183 for the conception of the germ-cells 

 supplied by my theory.) Experimental biology de- 

 scribes fertilization as the activation of the "ripe" 

 egg, that consists in the initiation of adequate chemi- 

 cal changes in the egg. (Concerning "ripe," see 

 p. 225.) 



In sexual generation both parents provide each a 

 single cell. Any variation from this — as, for example, 

 one egg giving rise to two individuals — is an ex- 

 ception. 



"Contact of the sperm," as Edmund B. Wilson 

 describes, "calls forth a powerful and almost instan- 

 taneous reaction by the egg that is responsible not 

 only for entrance of the sperm, but also for many 

 other changes in the ooplasm. "^^ 



The fusion of the ovum and the sperm-cell is a 

 process which involves a number of interesting and 

 important phases. 



Fertilization, impregnation, conception, having 

 taken place, immediately the process of cell division 

 commences. With the first division of the impreg- 

 nated ovum, the growth and development of the 

 new individual is begun. 



A number of biologists obtained some very interest- 

 ing results experimenting on fertilized eggs at the 

 two-cell stage. Jacques Loeb put the eggs of the sea- 



^* The Cell in Development and Heredity (1925), 409. 



