108 HEREDITY AND ENVIRONMENT 



developed ova and spermatozoa come to a full 

 stop before fertilization and they do not usu- 

 ally begin again until a spermatozoon has 

 entered an ovum, or until the latter has been 

 stimulated by some other outside means. In 

 some animals and plants eggs may develop 

 regularly without fertilization, the stimulus to 

 development being supplied by certain ex- 

 ternal or internal conditions; in other cases, 

 as Loeb discovered, eggs which would never 

 develop if left to themselves may be experi- 

 mentally stimulated by physical or chemical 

 changes in the environment, so that they un- 

 dergo regular development. The development 

 of an egg without previous fertilization is 

 known as parthenogenesis or virgin reproduc- 

 tion; if it occurs in nature it is natural par- 

 thenogenesis, if in experiments it is artificial 

 parthenogenesis. Natural parthenogenesis is 

 relatively rare and in the vast majority of 

 animals and plants the egg does not begin to 

 develop until a spermatozoon has entered it. 



But the spermatozoon not only stimulates 

 the egg to develop, as environmental condi- 

 tions may also do, but it carries into the egg 



