1 70 AGE, GROWTH, AND DEATH 



one might easily be led to the suspicion that there 

 could be no possible new young, a conclusion ob- 

 viously absurd, for there is a constant renewal of the 

 generations. Some device, therefore, must exist by 

 which that which is young is perpetuated, for that 

 which is old cannot again become young, and of that 

 device I should like to say something this evening. 



Formerly the notion was prevalent that under suit- 

 able conditions old cells could resume the young 

 state and undergo redifferentiation. Hans Dreisch, E. 

 Korschelt, 1 and a few other contemporary German 

 and American writers still regard the occurrence of 

 retrogressive development ^ Ent differ enzirung'^ as 

 credible. I have recently been over the few cases of 

 alleged evidence in favour of the notion in question, so 

 far as they are known to me, but in no case has it 

 been proved that a differentiated cell has changed 

 into an undifferentiated one. Provisionally, at least, 

 we can maintain that there is no exception to the law 

 that an old cell cannot go back in its development. 

 A most singular theory involving the assumption of the 

 redifferentiation of cells has recently been enunciated 

 by a German author, Kronthal, 2 who says that nerve 

 cells arise each by the fusion of several white blood 

 corpuscles (leucocytes] into a single mass. In answer 

 to him it may be said : firstly, that the origin of nerve 



1 E. Korschelt, Regeneration and Transplantation, Jena, 1907, Gustav Fischer. 

 See especially pp. 76 and 99. Compare also T. H. Morgan, "The Physi- 

 ology of Regeneration," Journal of Experimental Zoology, iii., p. 493 (1906). 



2 Kronthal, Von der Nervenzelle tin der Zelle ini Allgemeinen, 8vo, pp. 274, 

 (1902, G. Fischer, Jena). Compare also Anatomischer Anzeiger^ xxii., 448. 



