The time-graded regeneration field in planarians 



These are the chief facts so far discovered concerning the time-graded regeneration 

 field in planarians. 



As to the nature of the field I have only a few suggestions. It may be that the struc- 

 tural foundation is the nervous system; I do not think so, however, because there 

 seems to be no correlation between the pattern of the nervous system and that of the 

 field. More probable is the notion that the quantity of neoblasts determines the rate 

 of regeneration. Some observations support this view, others do not. The question 

 can of course only be answered by counting the neoblasts; this counting is being done 

 in my laboratory, but I can as yet give no figures. But even if the quantity of neo- 

 blasts were to determine the rate of regeneration, we are still ignorant of the factors 

 which determine a strict species-specific distribution of neoblasts, which otherwise can 

 easily wander freely about in the planarian body. Here it might be suggested that some 

 diffusable substance from the head ganglia attracts the neoblasts ; such a mechanism 

 might account for a caudally tapering concentration of the neoblast population. 



The facts so far revealed about the time-graded regeneration field have led me to 

 some considerations concerning three major problems in morphogenesis: polarity, 

 inhibition, and gene-action. 



POLARITY 



We perceive polarity in single cells and in whole multicellular organisms. This has 

 led some authors to postulate polarity in every single cell in the organism, but I 

 think without justification. In the intact planarian body the neoblasts cannot be 

 polarized. Neoblasts from a certain region may migrate to an anterior wound and 

 regenerate a head ; or they may migrate to a posterior wound and regenerate a tail- 

 tip; they may also migrate to a lateral wound and regenerate missing side parts of 

 the body. The neoblasts are totipotent. 



The neoblasts are compelled to start regeneration in a certain main direction; 

 the directive force must come from the remaining part of the body. For instance in 

 Planaria lugubris a blastema formed at an anterior-facing wound surface will always 

 start by making a head, irrespective of the level along the main axis of the body. In 

 the same way, a blastema at a caudal wound surface will always start by making a 

 tail-tip, also irrespective of level. Here polarity displays itself. Exceptions will be 

 dealt with later. 



What is the nature of the mechanism ? We know that the neoblasts forming the 

 blastema are totipotent. We know that in the intact body they are under control 

 and are inhibited from displaying their potentiality. But in an anterior blastema, for 

 instance, the neoblasts are freed from inhibition by the forepart of the body, which 

 was removed by the cut, but they are not freed from inhibition by the remaining 

 hindpart of the body. The same holds true — mutatis mutandis — for a posterior blastema. 



The blastema is to be compared with the anterior or posterior half of a blastula 

 not yet determined. Hence the two kinds of blastemata will act accordingly, that is, 

 they will start morphogenesis at opposite ends of the main axis, comparable with the 

 most animal and most vegetal part of the egg. I cannot help thinking of the well- 

 known double-gradient hypothesis of Runnstrom concerning the echinoderm egg. 

 There we have a close resemblance to the phenomenon dealt with here. 



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