The time-graded regeneration field in planarians 

 and some of its cy to-physiological implications 



by 



H. V. BR0NDSTED 



Instilutfor Aim. ^oologi, Kobenhavns Universitet 



THE TIME-GRADED FIELD 



Planarian regeneration has received much attention because these animals, especi- 

 ally the fresh-water triclads, seem to be 'immortal under the edge of the knife', to use 

 an impressive phrase of Dalyell (1814). 



Prominent biologists have sharpened their experimental arts and their scientific 

 wit in their efforts to solve some of the deep-rooted riddles immanent in the specta- 

 cular powers of regeneration exhibited by these inconspicuous animals. 



T. H. Morgan and C. M. Child have been among the foremost in this research 

 work, and some far-reaching hypotheses regarding morphogenesis in general have 

 emanated from this work, such as the well-known gradient hypothesis of Child. 



The tide of research on planarian regeneration has flowed and ebbed, as in most 

 other scientific disciplines. Now we are in a period of rising water, thanks in part to 

 the brilliant work carried out in Wolff's laboratory in Strasbourg. The two out- 

 standing results are (1) Wolff's and Dubois' (1947) and Dubois' (1949) demonstration 

 that neoblasts, totipotent embryonic cells, form the regeneration blastema and hence 

 are responsible for the rebuilding of the missing parts, and (2) Wolff's and Lender's 

 (1950, 1 951) finding that eye-formation in Polycelis depends on the presence of head 

 ganglia. Other lines of approach have been followed in my laboratory for several 

 years; one of these I should like to outline before you to-day. 



We know from the earliest experiments, notably those of Morgan (1902), that in 

 some planarian species every part of the body has the power to regenerate a whole 

 animal ; thus every part of these species is able to regenerate a head from an anterior 

 wound. We may therefore ask: why do not many heads regenerate from the anterior 

 wound of a transected animal? 



Figures 1 and 2 show some instances of heads formed at rather unexpected places; 

 in the second instance regeneration occurs in spite of the fact that the old head was 

 not cut away — which, incidentally, shows that no inhibitory force for head-regenera- 

 tion can travel through adult tissue. The heads at the 'arms' in Figure 1 and the head 

 in the window in Figure 2 are regenerated at a much lower speed than a median head 

 from an anterior transverse cut. This suggested that time was involved in regeneration 

 of only one head from a surface where the potentiality of plural head-regeneration 

 was present. 



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