REGENERATION 121 



This conception was taken to the extreme by Weisz (1954) 

 when he stated that size is not a Hiniting condition of regeneration 

 in protozoa and that theoretically one molecule of deoxyribonucleic 

 acid surrounded by a shell of cytoplasm should be able to re- 

 constitute the organism. He therefore regarded the explanation of 

 Lillie and Morgan concerning minimal size as untenable. Weisz 

 (1948a) found successful regeneration in pieces of coeruleus as 

 small as 70 /x in diameter; yet he reported and later emphasized 

 (Weisz, 1953, 1954) that even much larger fragments could be 

 produced which are incapable of regeneration. The crucial point, 

 he thought, was w^hether or not a fragment contains a portion of 

 the normal primordium site and hence presumably specialized 

 kinetosomes which alone can produce an oral primordium. Yet 

 this explanation is contradicted not only by Causin's (1931) 

 demonstration of the dispensability of the primordium site but also 

 by Weisz's own experiments, mentioned above, showing that 

 regeneration can occur in aboral halves, though much delayed. 

 And I have found (Tartar, 1958b) that nucleated primordium 

 sectors, or just the part of the stentor cell which contains the 

 primordium site, can regenerate completely, with mouthparts, 

 only if of sufficient size. 



When a sample of coeruleus is set aside for a week or two without 

 added nutrients the animals starve until individuals are produced 

 which are much smaller than normal daughter cells. Starting with 

 these starvation dw^arfs, I cut off substantial portions of the posterior 

 pole and found that pieces as small as 75 ft in diameter or only 

 I /123rd the volume of large, pre-starvation stentors, could re- 

 generate completely and survive for over 6 days (Fig. 29). Although 

 these tiny stentors had much fewer than the usual number of 

 membranelles, the width and length of these organelles when 

 measured proved to be very nearly the same as in large animals, 

 and these relatively oversized organelles caused the anterior end of 

 the tiny animals to shake and shudder with their beating. 



Therefore it seems to me, as previously suggested (Tartar, 

 1941b), that a Hmit to reconstitution of the normal form is imposed 

 simply by the fact that the units of ectoplasmic structure are each 

 of a nearly constant size or incapable of '' miniaturization ", so that 

 with decreasing volume there will come a point beyond which 

 the formation of anything like a normal set of feeding organelles 



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