146 organisers: inducers of differentiation 



•.e.v.- — ' ^ 



■ p.e.v. 



# 



p.e.v.., — ► 



i' 



M 



B 



D 



Fig. 68 



The regional inductive properties of the organiser and the regional reactive 

 properties of different levels of host- tissue in Urodele embryos. Photographs of 

 the embryos on which the diagram fig. 67 is based. A, Head-organiser grafted 

 at head level ; the secondary embryo (on the right) consisting only of a head with 

 ear- vesicles, and eyes fused with those of the primary embryo (f.ov.). B, Head- 

 organiser grafted at trunk level ; the secondary embryo (on the right) is nearly 

 complete but its anterior end is imperfect, it lacks eyes, and its ear-vesicles (s.e.v.) 

 are at a lower level than those of the primary embryo (p.e.v.). C, Trunk-organiser 

 at head level ; the secondary embryo (on the left) is complete, its cephalic struc- 

 tures (s.o.v. eyes, s.e.v. ear-vesicles) on a level with those of the primary embryo 

 (p.o.v., p.e.v.). D, Trunk-organiser at trunk level ; the secondary embryo (on the 

 right) consists only of a trunk, ending anteriorly with ear- vesicles on a level with 

 those of the primary embryo. (From Spemann, Arch. Entzumech. cxxiii, 1931.) 



