68 THE NATURE OF LIFE 



at a definite stage and to hold them at 

 this point for a long time; afterward they 

 can be made to finish their development. 

 In experiments on rats it is possible to 

 allow development to proceed on a nor- 

 mal diet and then by substituting a de- 

 ficient diet to stop development entirely. 

 In this case there is no starvation since 

 the animals do not lose weight. They 

 remain undeveloped and may be kept 

 in this condition long after their brothers 

 and sisters have grown up and had fam- 

 ilies of their own. If now the normal 

 diet be substituted for the deficient one 

 development proceeds normally. 



Thus it is also possible to reverse de- 

 velopment and cause certain animals 

 (flat worms) to grow smaller, until they 

 are not as large as when hatched from 

 the egg. During this process they lose 

 some of their organs, returning toward 

 the infantile condition. They can then 

 be caused to develop normally, after 

 which development can again be re- 

 versed, and so on. 



