organisers: inducers of differentiation 



Fig. 82 



Organising action of engrafted heads in Planaria. 

 ■ a, A lateral post-pharyngeal graft has induced a 

 lateral outgrowth and a secondary pharynx, c, A 

 sub-terminal graft has caused a reversal of polarity 

 in the terminal portion of the host, and has induced 

 two secondary pharynges. d, A terminal graft has 

 induced a single secondary pharynx with reversed 

 polarity ; tlie region of reversed polarity is capable 

 of considerable autonomy of movement (dotted 

 outline), b, A sub-terminal graft has induced two 

 secondary pharynges, and a marked reversal of 

 polarity in the host's terminal region, a, b, c, 

 heteroplastic grafts, d, homoplastic graft. (From 

 Santos, Biol. Bull, lvii, 1929.) 



