2S8 SENESCENCE AND REJUVENESCENCE 



latter group is that described by H. C. Miiller, of the dedifferentia- 

 tion after isolation and mutilation of the parts which bear the 

 sexual organs — the so-called gonophores — of certain hydroids into 

 masses of embryonic cells which give rise to stolons and so may pro- 

 duce new vegetative, asexual colonies. Dedifferentiation occurs 

 in the reduction by starvation of planarians (E. Schultz, '04). The 

 parenchymal cells of Planaria, which play the chief part in the 

 formation of new tissue in regeneration, are certainly to all appear- 

 ances differentiated cells and undergo dedifferentiation when they 

 begin their growth as new tissue. In the tapeworm Moniezia the 

 sex cells may arise by the dedifferentiation of parenchymal cells 

 (see pp. 331-32). The return of old, flat ectoderm cells to the 

 embryonic condition has been observed by Romer ('06) in the 

 regeneration of bryozoa. Krahelska ('13) has described the 

 dedifferentiation of the albumen gland in certain snails during 

 oviposition. In the remarkable reduction of the branchial region 

 in isolated pieces of the ascidian Clavellina, which represents a 

 return to the condition of a bud in an early stage of development, 

 extensive dedifferentiation of cells certainly occurs (Driesch, '02; 

 E. Schultz, '07). Schaxel ('14), however, maintains that in this 

 case the differentiated cells are lost and the new parts arise from 

 undifferentiated cells which remain, but his assumption that the 

 cells which take part in the new development are undifferentiated 

 is not proved. In the regeneration of the lens of the eye in 

 amphibia the cells of the iris which give rise to the new lens very 

 evidently undergo dedifferentiation (G. Wolff, '95; Fischel, '00). 

 Numerous other cases of more or less complete dedifferentia- 

 tion have been more or less closely observed and described and 

 doubtless many others still remain to be described in cormection 

 with agamic reproduction, reconstitution, and even in the normal 

 life of organisms. The changes in gland cells during their cycle of 

 activity (pp. 189-91) and various other periodical changes also 

 belong in this category. But the morphological criterion of reju- 

 venescence is at best unsatisfactory, for it is merely a rather unre- 

 liable indicator of the physiological condition of the cells. As is 

 evident from the experimental study of the developmental stages 

 of many animals, cells may undergo considerable changes in the 



