332 



SENESCENCE AND REJUVENESCENCE 



disappear, and the nucleus begins to divide, apparently at first 

 amitotically. 



In some of the lower animals new individuals arise agamically 

 or can be produced by 

 experimental isolation of 

 pieces from regions of the 

 body which do not contain 

 sex organs, yet these indi- 

 viduals are capable of pro- 

 ducing sex cells. To 

 assume that these regions 

 of the body contain germ 

 plasm in the Weismannian 

 sense ready to develop into 

 ovaries or testes when 

 necessary is simply to beg 

 the question. To all 

 appearances germ cells de- 

 velop in such cases from 

 more or less differentiated 

 cells of the region in- 

 volved by a process of 

 dedifferentiation and re- 

 differentiation, and the 

 assumption of a pre- 

 existent germ plasm is 

 entirely unnecessary. 



It is scarcely probable 

 that the germ plasm is a 

 totally different thing in 

 animals and plants. In 

 the preceding section it 

 has been pointed out that 

 for a very large number of 

 plants the development of 



germ cells from differentiated functional cells of the plant body 

 has been experimentally demonstrated. This fact in itself creates 



Figs. 146, 147. — Formation of a testis from a 

 muscle cell in Moniezia: Fig. 146, large muscle 

 cell with single fiber; Fig. 147, transformation 

 of muscle cell into testis. 



