GROWTH AND DIVISION 85 



-In de Terra's study on coeruleus it was demonstrated that during 

 fission the uptake of radiophosphorus (P^") dropped to one- 

 twentieth its rapid predivisional rate and was in fact the same as 

 that of enucleated dividers from which the compacted macro- 

 nucleus had been removed. This indicates that the large nucleus 

 is not very active biochemically at the time of fission and thus 

 helps to explain why division can continue to completion in its 

 absence. 



Extension of studies on persisting division called for operations 

 on still earlier stages of dividers. In recent tests yet unpublished I 

 found that division in Stentor coeruleus can go ahead after some 

 rather drastic operations and often when the division process is 

 by no means nearing completion. Dividers in stages 2 to 5 were 

 cut in two transversely but the two halves allowed to heal in place. 

 The primordium also rejoined its parts and division could be 

 consummated in a perfectly normal manner. Stage 4 dividers were 

 cut longitudinally and the oral half rotated 180° on the other in 

 heteropolar orientation; furrow formation still occurred and 

 division was nearly complete although the division products did 

 not separate (Fig. 19A). The mouthparts of dividers in stages 2, 3 

 and 4 were excised and the cells either split down the back and 

 opened out flat or cut and spread out in three parts like a clover 

 leaf, and still division often followed, yielding proters which 

 regenerated the missing mouthparts later. Isolated longitudinal 

 halves of dividers cut before there is any visible sign of a furrow 

 (stage 5) could cut the striping and form furrows. Usually the 

 fragment did not actually separate into two pieces, as others have 

 also found ; yet in two cases at the preceding stage 4, longitudinal 

 oral halves did complete division. These tests clearly indicate that 

 the division process is not so delicate and precisely adjusted that 

 disturbances cause its undoing. 



The oral primordium was excised from dividers as early as 

 stage 3 and the animals continued division. Because the headless 

 opisthes later formed oral structures through a regeneration 

 primordium we can infer that the macronucleus was also divided. 

 Well-formed primordia at stage 4 were also circumscribed and 

 rotated 180° in situ and subsequent fission was still successful. 

 These experiments, as well as certain of the aforementioned, 

 show that although the fission line begins at the anterior end of the 



