212 AGE, GROWTH, AND DEATH 



that the regeneration is accomplished by the growth 

 of undifferentiated cells, which subsequently undergo 

 differentiation. 



The cases of regeneration which we have re- 

 viewed are all connected with the repair of injuries. 

 But there are also cases known of spontaneous and 

 normal regeneration, as they might be called. For 

 example, there are certain fresh-water jointed worms, 

 annelids, which produce in the midst of their body 

 from time to time a budding zone, a narrow band of 

 tissue intervening between two segments. The ante- 

 rior part of the zone forms a new tail for the anterior 

 part of the worm, the posterior part of the zone 

 forms a new head for the posterior part of the worm ; 

 division follows, and thus out of one worm two are 

 produced. The external features of this wonderful 

 process were described very well indeed in 1771, by 

 the celebrated naturalist O. F. Miiller. 1 It was re- 

 served for Carl Semper, 2 under whom I had the hon- 

 our to study in 1875-76, to demonstrate, at that very 

 date, that the cells of the budding zone are of the 

 embryonic type, and that after having multiplied 

 sufficiently they begin to differentiate into the tissues 



und iiber das Verhaltniss der Regeneration und der Knospung zur Keimblatt- 

 lehre," Jena 'ische Zeitschrift, xxxiii., 263-344, Taf. XII., XIII. (1899). For 

 our present purposes it is a matter of regret that on the cytological side 

 Schultze's observations leave much to be desired. 



1 O. F. Miiller, Naturgeschichte einiger Wurmarten des sussen und salzigen 

 Wassers, Kopenhagen, 1771. 



2 Carl Semper, " Die Verwandschaften der gegliederten Thiere, III." Sem- 

 per's Arbeiten, Zool. Zootom. Inst. Wiirzburg, iii., 115. See especially 

 p. 



