organisers: inducers oe differentiation 



65 



indicate that the organiser in Coiymorpha is not a specific tissue or 

 structure, but any level of the stem will act as an organiser, although 

 pieces from distal levels are more 

 potent. The bearing of these facts on 

 the theory of gradient-fields and the 

 interpretation of the mode of action of 

 organisers will be discussed at greater 

 length in Chap, viii (fig. 138). 



In Platiaria, the head of one worm 

 grafted into the posterior region of the 

 body of another induces the formation 

 of a pharynx and brings about the re- 

 organisation of the host-tissues so as to 

 make them conform to the new polarity 

 set up by the graft. Here again, the 

 effect is not species-specific, for a head 

 of Planaria dorotocephala will act as an 

 organiser in the tissues of Planaria 

 maciilata^ (figs. 81, 82). 



These last experiments merely extend 

 previous w^ork on regeneration in Plan- 

 arians and various worms. In Planaria, 

 for instance. Child had shown ^ that the 

 reorganisation of the old tissues of a 

 posterior fragment, of which the most 

 obvious eflFect is the production of a 

 new pharynx, only occurs if a head is 

 regenerated. He further showed, in ex- 

 periments where the size of the re- 

 generated head was varied and controlled 

 by the use of anaesthetics in varying (From Mutz, Arch 

 concentrations, that the size of the new '"^^^'- ^xxi, 1930.) 

 pharynx and its distance from the anterior end of the piece were 

 correlated with the size of the regenerated head (see Chap, viii, 

 pp. 287, 290). 



As noted in Chap, viii (p. 288) the head segments of the Poly- 

 chaete worm Sabella act during regeneration as an organiser capable 

 ^ Santos, 1929. 2 See Child, 1915 a, pp. 102, 138. 



Fig. 80 



Organiser grafts in Hydra. 

 Bud (k) induced from stock 

 {b) by grafting an oral end of 

 another individual {a) on to 

 the aboral end of the stock. 

 Entw- 



