490 J. S. HUXLEY AND G. R. DE BEER 



actively (no division normtilly occurs in adult smooth muscle). 

 If such a preparation be grafted back into its former position 

 it is just possible to arrest the mitoses, but redifferentiation 

 proceeds no further (Champy, 1913). (Strangeways informs 

 us verbally that he has been more successful.) On the other 

 hand, redifferentiation of dedifferentiated tissue has been 

 obtained by Drew (1 923) in vitro. 



Eedifferentiation of the zooid was not obtained in Obelia as 

 it was in Clavellina (Driesch, 1906 ; Huxley, unpub.), Pennaria 

 (Cerfontaine, 1902), or Sycon (Huxley, 1911). But in those 

 cases where redifferentiation does occur, we must ask whether 

 the new adult structure is formed from the redifferentiation of 

 the dedifferentiated cells, or from indifferent cells which have 

 all along retained the full hereditary potentialities. The study 

 of budding and asexual reproduction, especially Hadzi's work 

 (1910) on Hydra, suggests that the latter is usually the case. 

 If this is so, then the failure to redifferentiate is in no way due 

 to the inactivity of the dedifferentiated elements, but of the 

 indifferent cells. Vandel (1921), however, shows that in 

 regenerating Planarians (Polycelis), the new pharynx is pro- 

 duced from cells of other organs which dedifferentiate and then 

 redifferentiate along new lines, scarcely any mitoses being 

 observed. This is a good example of p 1 u r i p o t e n t dedifferen- 

 tiation (Adami and Macrae, 1914 ; Huxley, 1921 ft). The usual 

 process, however, in colonial forms, is for the dedifferentiated 

 tissues to provide material for new outgrowths of the nature 

 of stolons, from which later new zooids may arise (cf. Ascidians, 

 Huxley, 1921 b ; Hydroids, Miiller, 1913). 



When the metabolic requirements of the zooid have decreased, 

 the equilibrium between it and the stem is upset and the 

 balance is now in favour of the latter (Huxley, 1921 h). In 

 the higher animals complete resorption of systems does not 

 usually occur. For one thing the cells are too solidly packed 

 in tissues, and they are usually attacked by phagocytes before 

 they have even had time to be resorbed. But a concurrence 

 of both processes is seen in the absorption of the tail in Ascidian 

 tadpoles. Here, according to Delage and Herouard (1S98), ' ses 



