320 



PRINCIPLES OF EMBRYOLOGY 



already existing organs. We have thus httle sign of anything correspond- 

 ing to the phenomenon of dominance in hydroid regeneration. 



There are, however, some inductive interactions in operation m plan- 

 arian regeneration. If the head region oi Dugesia lugubris is grafted into 

 the posterior of the body, behind the pharynx, it can mduce the formation 

 of a supernumerary pharyngeal region from its neighbourhood; while 

 in PolyceUs nigra, the cerebral ganghon induces the formation of eyes 



Figure 14.5 



AB C D, staaes in the regeneration of two heads, following the T-shaped 



cut sho-^ in A; E, F, regeneration of a head following a longitudinal cut. 



(After Beissenhirtz, from Br0nsted 1946.) 



though only from a limited competent area which extends from the 

 anterior to the level of the pharynx (work of Sengel and Lender see 

 review of Wolff 1953). Wolff has summarised the conclusions from these 

 experiments in a diagram which is reproduced as Fig. 14-6. The first 

 event m a blastema engaged in head regeneration is the formation of the 

 ganghon; this induces a cephalic region (which in many forms includes 

 eyes); this region induces a pre-pharyngeal region, and that again a 

 pharyngeal region; and in the latter the pharynx itself eventually appears. 

 These inductive relations can be regarded as the expressions of an mdividu- 

 ation field which is most powerful when it arises within a blastema, since 

 it can then affect even the anterior part of the body, but which persists 

 at a lower intensity even in the fully developed organism. 



