-74 THE BIOLOGY OF STENTOR 



macronucleus can divide within the single cell, the separated parts 

 then rejoining. Yet it has been found (Popoff, 1909; de Terra, 

 1959) that if cell division is unequal the macronucleus is likewise, 

 quite as if this nucleus were passively pinched in two at the rod 

 stage by the constricting furrow (Fig. 15c). The proter is now 

 connected with the opisthe only by the tail pole, still attached at 

 the aboral end of the latter's membranellar band ; and up to this 

 time the daughters have continued to coordinate their backward 

 and forward swimming together (Gruber, 1886). Final separation 

 seems to be due to a twisting apart which sunders the fine con- 

 nection between the two cells. 



According to Johnson's account the micronuclei swell and 

 undergo mitosis after macronuclear division is completed, i.e., 

 within the essentially separate daughters. Each daughter thus 

 achieves about the same number of micronuclei as the parent cell. 

 After completing nodulation there are about the same number of 

 macronuclear beads in each product as there were in the original 

 animal. Therefore these nodes are half the size of the original ones 

 and nuclear growth consists largely of increase in the size of the 

 new nodes, though occasionally one segment may later divide in 

 two. This doubUng of the nodes of the macronucleus was first 

 noted by Balbiani (1882) and later confirmed by Johnson; Stolte; 

 and Tartar (1959c). It is therefore plausible that the macronucleus 

 clumps together to make possible its renodulation at once into 

 twice the original number of nodes. 



De Terra's (1959) studies on coeruleus have shown that the 

 uptake and incorporation of radiophosphorus is very rapid before 

 division but drops to one-twentieth of this rate when the macro- 

 nucleus is compacted and fission is in process, indicating that 

 nuclear increase does indeed occur by growth of the nodes and 

 not when the nucleus is in the coalesced stage immediately 

 preceding fission. 



The time required to complete the act of division is probably 

 quite variable but about 6 hours would be a reasonable average. 

 The first stages having to do with primordium formation proceed 

 more slowly. Fusion of the nodes of the macronucleus can occur 

 in one hour according to Johnson and the nuclei renodulate as 

 rapidly. Timing of the complex events in division presents special 

 problems in the integrated action of the cell, and one possibility 



