REGENERATION 



319 



numerous and complicated experiments which have been made in this 

 field. 



It is important to note that it is only regions of the old body in the im- 

 mediate neighbourhood of the cut which exert an influence on the course 

 of regeneration. Bronsted (1939) has isolated a section of the body anterior 

 to the pharynx by two transverse cuts (in Euplanaria luguhris) and then 

 grafted the anterior tip containing the original head with reversed polarity 

 to the posterior edge of the isolated segment. The grafted head in this 



Figure 14.4 



On the left, the head-regeneration field in the planarian BdcUocephala punctata. 



On the right, a head regenerated in a 'window' cut in the anterior region. 



(From Bronsted 1946.) 



situation exerted no apparent influence on the regeneration at the anterior- 

 cut surface, which proceeded to form a head exactly as it would have done 

 normally. Similarly, if a window is cut in a planarian, as in Fig. 14.4, the 

 existing old head does not inhibit the regeneration of the new head in the 

 window. Again, Raven and Mighorst (1948) have shown that the rate of 

 head formation at a given level in Euplanaria lugubris is not in the slightest 

 affected by the presence or absence of a further posterior cut at which, if 

 it is present, a tail will simultaneously be regenerating. Thus in planarians 

 there is little or no evidence of competitive interaction between the two 

 ends of an isolated segment, or between a regenerating surface and 



