150 



organisers: inducers of differentiation 



an old organiser is grafted, reversed, into the antero-ventral region 

 of the host.^ 



Those cases in which the secondary embryo fails to adapt itself 

 to the polarity of the primary embryo are of interest because 

 certain of the paired structures of the secondary embryo, such as 

 ear-vesicles, lie at different levels in the host. In these cases it is 



Fig. 71 



Section through an organiser- graft in Triton, in which the anterior end of the 

 secondary embryo lay at right angles to the long axis of the primary embryo. The 

 left ear- vesicle of the secondary embryo, l.sec.a.v., which lies nearer the anterior 

 end of the host embryo, is larger than the right, r. sec. a. v. pr.br. brain of primary 

 embryo; g.c. gut cavity; sec.br. brain of secondary embryo. (After Spemann, 

 Arch. Entzomech. cxxiii, 193 1, simplified.) 



found that the vesicle nearer to the anterior end (animal pole) of 

 the host is larger than the other, and this shows that there is in the 

 tissues of the host a stratification of capacities to react to the 

 organiser (figs. 70, 71 ; and see pp. 147, 319). 



In addition to the regional difference between head-organiser 

 and trunk-organiser, it seems, however, that (contrary to previous 



^ Lehmann, 1932. 



