H. V. BR0NDSTED 



If we cut out a narrow transverse section from the middle part of the planarian 

 body, we always get a head anteriorly and a tail posteriorly; if we imagine the narrow 

 section made so short as to vanish but still having neoblasts available, we should find 

 that we had a cluster of totipotent cells in competition, the anterior half of them 

 'animalized', the posterior half 'vegetalized', and then the double gradient of Runn- 

 strom would be at work. 



Now, every comparison has its weak point, and this one too. In the planarian 

 blastema the polarity is imposed by the remaining adult body; in the egg the polar- 

 ity is imposed on the oocyte in a manner as yet unknown. But the analogy shows us, 

 I think, that the polarity of the planarian body is a structure derived directly from 

 the oocyte. 



Here the time-graded field enters into the discussion. 



The Janus-head or 'Janus-tail' has always puzzled morphogenetecists. We know that 

 polarity may be reversed by external stimuli in hydroids : oxygen, pH, light, electric 

 current, etc. But why should very short transverse segments from the forepart of the 

 planarian body often regenerate heads at both anterior and posterior surfaces ; and 

 why should transverse segments from the hindpart often regenerate two tails ? 



It should be emphasized that only very short segments regenerate Janus-heads 

 or Janus-tails. It should also be pointed out that Janus-heads are made only in the 

 anterior part of the body, Janus-tails only in the posterior part, and neither of them 

 in the mid-part. It must further be borne in mind that in an ordinary transverse 

 segment from the forepart the head-blastema is the first to be differentiated, but 

 in a posterior segment the tail-blastema. Therefore — again as in the echinoderm 

 egg — the forepart is more 'head-minded', the posterior part more 'tail-minded'. 

 This is probably due to the structure of the nervous system. 



A very narrow transverse segment from the forepart contains a part of the time- 

 graded field with a high head-building rate. The totipotent cells form the anterior 

 and posterior blastemata almost equally fast. I suggest that the speed in the fore- 

 part of the time-graded field is so great that the totipotent neoblasts have had time 

 to direct themselves into head-formation, also at the caudal wounds, before inhibition 

 from the very small remaining part of adult tissue has got a chance to inhibit, the 

 more so as such a narrow strip of tissue is nearly disarranged by the massive migra- 

 tion of the neoblasts. The same may of course be said — mutatis mutandis — of posterior 

 narrow sections giving rise to Janus-tails. In mentioning the manifestation of polarity 

 in the blastema I have not used such words as organization, organization forces, 

 induction and the like. I must admit that I am not very enthusiastic about such 

 notions. It seems strange to me that every level of the body should be able to 'organ- 

 ize' or 'induce' head-formation in an anterior blastema; and moreover that every 

 level should be able to discern whether to make a head or a tail. It seems more prob- 

 able to me that lack of proper inhibition gives the blastema its main direction of 

 determination. 



INHIBITION 



Wolff and Lender (1950) and Lender (1951) have shown that the first structures to be 

 regenerated in an anterior blastema are the head ganglia. Ribonucleic acids are 

 conspicuous elements in the nerve cells; RNA is also a conspicuous element in the 



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