The time-graded regeneration field in planarians 



neoblasts; they are strongly stained by pyronine and by Einarson's gallocyanine. It 

 has been shown by my wife and myself that RNA accelerates head-regeneration in 

 starved planarians. (A. and H. V. Brondsted, 1953). All this opens up certain 

 possibilities concerning the mechanism of morphogenesis in the planarian blastema. 

 Now Brachet has proved that RNA accumulates in the dorsal part of the amphibian 

 germ. This led me to suggest that the vertebrate germ may well be regarded as a 

 time-graded morphogenetic field analogous with the regeneration blastema. 



We know from a great number of experiments that a vast assortment of stimuli may 

 evoke neurulation in the ventral epidermis of the young amphibian gastrulae; 

 moreover, Barth (1941) has shown self-differentiation in explanted ventral ectoderm; 

 this proves that the ventral epidermis has potencies of the same kind as the dorsal 

 one. Could it be that normally it does not reveal these potencies because its rate of 

 cytoplasmic reactivity is too slow, so that inhibition from the dorsal parts suppresses 

 its tendency to differentiate? It may well be, in induction experiments, that contact 

 with presumptive entomesoderm and with various metabolites accelerates its activi- 

 ties so as to reach so high a level of reactivity towards gene-action and hormones 

 produced by gene-activity as to be able to escape inhibitory influences. This hypo- 

 thesis might be tested experimentally in explants. 



The current hypothesis, worked out by Holtfreter, Brachet and others, is that 

 some sort of blockage exists in the ventral epidermis ; if the blockage is removed then 

 neurulation follows. I think this hypothesis is right, and I only suggest that the block- 

 age is imposed on the ventral epidermis by the inhibitory influence from the far- 

 advanced dorsal epithelium, the 'high-point'. This reasoning leads to a working 

 hypothesis of a serological nature. 



Long ago I suggested (Brondsted, 1936) a mechanism for the sorting out of cell 

 types in the reconstitution bodies of freshwater sponges pressed through bolting silk. 

 Four main cell types could be discerned in the dissociated cell material. The results 

 were confirmed shortly after by Brien (1937). The problem was to elucidate how the 

 various cell types regained their proper situations so that a working system could be 

 re-established. 



While watching the movements of the isolated cells at the bottom of the dish I 

 very often saw a curious phenomenon : when two cells met, then one or both might 

 wriggle and send out pseudopodia at a far greater speed than when not in contact; 

 the phenomenon was photographed by the time-lapse technique. Moreover, when 

 two cells touched one another, one of them might throw out a pseudopodium with 

 explosive force, as shown in Figure 20. 



Here, surely, one might speak of negative cytotaxis. I therefore suggested that 

 such movements, arising from mutual incompatibility between different cell types, 

 might well be conceived as morphogenetic movements leading to reassortment. I 

 regret that I have had no time to pursue this very promising problem. It was 

 therefore with much pleasure that I found that Holtfreter later (1947) adopted 

 similar views. 



I think that this problem is connected with far-reaching lines of research concern- 

 ing serological differences arising during early embryogenesis. I have not time here 

 to go deeper into the matter. A very comprehensive and clear survey has been given 

 by Woerdeman (1953). 



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