REPRODUCTION 27 



direct experimental evidence on this 

 point. 



After the fusion of gametes to form 

 the fertilized egg, development proceeds 

 by growth and cell division mitil an 

 adult organism is produced. One of 

 the important tasks of biology is to 

 explain the mechanism of development. 

 It is possible to control development to 

 a great extent by physical and chemical 

 agencies. It would lead us too far to 

 enumerate the triumphs of this fascinating 

 field of work. It will suffice to mention 

 Loeb's discovery of artificial partheno- 

 genesis by which the egg can be made to 

 develop without fertilization. Usually 

 this involves chemical treatment but in 

 the case of the frog it is sufficient to prick 

 a hole in the membrane which surrounds 

 the egg. The egg then develops, with- 

 out fertilization, into a normal adult. 



By another treatment the egg can be 

 made to begin to develop inside out, with 

 the internal organs on the outside of the 

 body. 



