PROCESS OF REPRODUCTION IN ORGANISMS. 13 



cells in certain cestodes may take part in producing germ cells 

 (Child, '06). 



In plants buds, which later give rise to germ cells, may arise 

 from parts which have already undergone some degree of dif- 

 ferentiation, but which under altered conditions dedifferentiate 

 and produce buds. 



How shall we interpret all these and many other facts of the 

 same kind? Shall we conclude that actual observation counts 

 for nothing and that because these apparently differentiated 

 cells produce germ cells they must have contained undifferen- 

 tiated germ plasm at all times? This is nothing more nor less 

 than an illogical procedure. We are bound to accept our ob- 

 servations until we have some actual evidence which conflicts 

 with them. And not only is there no evidence to prove that cells 

 cannot dedifferentiate and redifferentiate, but there is abundant 

 evidence to prove that many cells can do this. The data upon 

 this point concerning plant cells are numerous and convincing 

 and the occurrence of dedifferentiation has been generally 

 accepted by the botanists as a fact for many years. 



In the Protozoa the soma itself or a part of it becomes the 

 gamete. We may of course assume that undifferentiated germ 

 plasm is continuously present in the nucleus or micronucleus, 

 but if this is the case why is not the protozoon always a gamete? 

 Why does sexual reproduction or conjugation in these forms occur 

 only periodically? Evidently it does occur only when the 

 organism is in a certain physiological condition. How are we 

 to account for this fact if the germ plasm is continuously present 

 and independent of the soma? 



In both plants and animals the production of gametes is 

 associated with a certain stage of the life history. Even if the 

 primitive germ cells appear in early embryonic life they do not 

 develop into gametes until the growth period is completed. 

 In many plants and some animals it is possible to induce experi- 

 mentally the development of gametes and the conditions which 

 bring about the sexual stage in these cases are conditions which 

 decrease metabolism and bring the organism into a state physio- 

 logically similar to more or less advanced old age. The extensive 

 work of Klebs and others on plants affords the strongest evidence 



