AGE CYCLE IN PLANTS AND LOWER ANIMALS 257 



And, lastly, the replacement of old by young cells in the body of 

 the animal also delays the senescence of the organism as a whole. 

 This process occurs more or less widely in all multicellular animals, 

 and in many of the lower forms it occurs to a very considerable 

 extent and more or less generally throughout the body. The old 

 cells or parts die and are either cast off or resorbed and replaced by 

 younger cells. In such cases senescence and even death are occur- 

 ring at all times, but the replacement may keep pace with the 

 aging and death of cells, so that the organism as a whole does not 

 grow old. Conditions in these forms are somewhat similar to those 

 in the higher plants discussed in an earlier section of this chapter, 

 where certain parts of the plant remain embryonic and give rise 

 more or less continuously or periodically to the various organs 

 which undergo senescence and death. In all cases of this sort cel- 

 lular reproduction is of course concerned and is unquestionably the 

 essential factor in the maintenance of an age equilibrium or retarda- 

 tion of senescence in the organism as a whole. 



The occurrence in animals of morphological rejuvenescence, i.e., 

 of dedifferentiation, has often been denied, but such denials are 

 based primarily rather on theoretical considerations than upon 

 observation. There can be no doubt that dedifferentiation occurs 

 extensively among the lower animals. The dedifferentiation of 

 protozoan cells has already been mentioned, and concerning those 

 cases there is no room for doubt that the morphological differ- 

 entiation disappears and reappears in the same cell. E. Schultz 

 ('08) and J. Nusbaum ('12) have brought together many cases of 

 dedifferentiation from the literature of the subject and have dis- 

 cussed their significance in a general way. It is impossible here to 

 do more than refer very briefly to a few of the well-established 

 instances of dedifferentiation. As regards the sponges, various 

 authors have described the occurrence of dedifferentiation of at 

 least some of the cells of the body under different conditions, such 

 as absence of hme salts, starvation, and dissociation of cells, and 

 there seems to be no doubt that extensive dedifferentiation may 

 occur in hydroids also.^ One of the most interesting cases in the 



' See, for example, on sponges: Bidder, '95; Maas, '06, '07, '10; Masterman, '94; 

 K. Miiller, 'iia, 'iib, 'iif; H. V. Wilson, 'iia; on hydroids: Berninger, '10; H. C. 

 MuUer, '13, '14; H. V. Wilson, '116. 



