1 66 



organisers: inducers of differentiation 



of transforming more posterior segments from the abdominal to the 

 thoracic type.^ Similar facts have been noted for the Oligochaete 

 Stylaria} These results will be further considered in connexion 

 with gradient-fields (fig. 137). They are of great importance in 

 showing that the processes at work in the organiser phenomena in 

 the early stages of Vertebrate development are similar in essentials 

 to those operating throughout life in regeneration and grafting 

 experiments in lower forms.^ There are, however, certain differences, 

 in that the vertebrate organiser works mainly by contact, whereas 



3 5 



Fig. 81 



Head-grafts in Planaria. 1, 2, 3, showing portions employed as grafts; 4, 5, 

 isolated fragments of the type of i and 3 respectively, 12 days after operation. 

 (From Santos, Biol. Bull. LVii, 1929-) 



these invertebrate organisms can effect a reorganisation of tissues 

 at a distance. For a further discussion of this point, see Chap, viii, 

 p. 310. 



Organiser phenomena in normal ontogeny, though again of a less 

 specialised type than in Amphibia, have been found in Echino- 

 derms. A curious result (referred to in Chap, v, p. 102) of the 

 isolation of animal halves of eggs and blastulae of the sea-urchin 

 Paracentrotus, is that such halves do not develop a stomodaeum if 

 they have been isolated from their vegetative counterpart at a 

 stage earher than 20 hours after fertilisation. A stomodaeum is, 



1 Berrill, 1931. " Harper, 1904. ^ See Child, 1928 c, 1929 a. 



