BEHAVIOR AND FUNCTIONS OF THE NUCLEUS 289 



few cases in which this has been done in dividers, subsequent 

 nodulation was inhibited (unpubUshed). The nucleus extended as 

 a rod but remained as such without any further change until 

 regeneration was induced by excising the mouthparts, whereupon 

 the normal nucleus was recovered (Fig. 80c). Therefore, although 

 nodulation may be intrinsic, the stimulus for it is given by the 

 cytoplasm during the last stages of oral redifferentiation. 



As previously discussed in the chapter on division, daughter 

 cells have about the same number of nodes as the parent (see Fig. 

 78c). Therefore coalescence of the macronucleus may be necessary 

 for its rapid renodulation into twice the number of nodes of about 

 half the size of the originals during fission. 



4. Equivalence of macronuclear nodes 



Nuclear fusions and clumping were also simply explained as a 

 means for recovering uniformity in parts which have become 

 diverse. In substantiation of this presupposition, Weisz (1949c) 

 claimed that an intranuclear difference does regularly develop 

 during divisional and reorganizational cycles in coeruleus, posterior 

 nodes (but not posterior cytoplasm) becoming increasingly incap- 

 able of supporting regeneration or even of maintaining oral and 

 caudal organelles as they approach the time of coalescence. Fusion 

 was then said to restore normal potency throughout the renodulated 

 macronucleus. Development and obliteration of differences seemed 

 to be correlated with degree of polymerization of nuclear DNA as 

 tested by methyl green staining (Weisz, i95o.b), though the reli- 

 ability of this determination was later questioned (1954). 



A similar development of intranuclear heterogeneity was 

 proposed for Blepharisma (Weisz, 1949), but in this case the mid- 

 portion of the macronucleus which no longer supported regenera- 

 tion was destined for extrusion and dissolution anyway. The 

 general conception is brought further into question by Weisz's 

 interpretation that proximity to organelles is the basis for main- 

 tenance of potency in adjacent nodes; for the mouthparts in 

 Blepharisma are near the level of the mid-nodes which should 

 therefore retain their full capacities ; and in Stentor, the posterior 

 nodes should support maintenance and regeneration of the hold- 

 fast, contrary to Weisz's own account. Since even enucleate frag- 

 ments can regenerate the foot (Tartar, 1956c), it is indicated that 



