164 organisers: inducers of differentiation 



§6 



Attention may now be turned to Invertebrates, and the question 

 naturally arises whether regions with similar or comparable organ- 

 ising capacities xist among them. This is found to be the case, 

 although the details, not unnaturally, vary considerably. 



In Hydro, the hypostome, or region surrounding the mouth, is 

 an organiser of simple type. When grafted into the proximal end 

 of another polyp it induces the formation of tentacles. It further 



Fig. 79 



Organiser grafts in Hydra. Induction of a bud by grafting an oral end of one 

 individual {a) into the flank of another. The polarity of the bud is the reverse of 

 that of the graft. (From Mutz, Arch. Entiomech. cxxi, 1930.) 



causes an outgrowth of host-tissues in which the original polarity 

 is overridden, and a new polarity established in relation to that of 

 the graft. ^ Although grafts of organisers in Hydra between different 

 species rarely succeed, an organiser from Pehnatohydra has been 

 found to produce an inductive effect in Hydra (figs. 79, 80). 



In another Coelenterate, Cbrymorpha, pieces of stem have the 

 power of inducing the formation of new polyps when grafted into 

 other stems." This case is particularly interesting, for the facts 



^ Browne, 1909; Rand, Bovard and Minnich, 1926; Mutz, 1930. 

 ^ Child, 1929 B, 1932. 



