72 THE BIOLOGY OF STENTOR 



Continuing its development, the posterior end of the primordium 

 begins to coil inward to form the gullet (stage 6). At this time the 

 cell usually shows a central contraction tending slightly toward a 

 dumbbell shape, but this constriction is not coincident with the 

 future furrow (Johnson) and makes its appearance earlier, as our 

 figure shows. 



A fission line then appears at both sides of the anterior end of 

 the primordium. To the right it cuts off the presumptive frontal- 

 field striping and runs approximately perpendicular to these stripes 

 which have been somewhat distorted by the movements of the 

 anlage. To the left, the furrow runs sharply posteriorly while 

 cutting obliquely across the wide granular stripes in this area, the 

 two ends of the fission line moving more and more transversely as 

 they proceed around the cell to meet on the lower dorsal side. By 

 being oblique, the furrow can cut the primordium, which runs far 

 anteriorly, into the posterior cell and yet divide the parent into 

 approximately equal daughters. The fission line is made evident 

 by a change in the pigmented stripes which leaves a colorless band 

 across each one. Possibly this may be caused by the formation of 

 new transverse contractile structures, pushing the granules aside 

 and later responsible for constriction at the furrow. 



Only when the membranellar band is fully formed and the gullet 

 begins to develop (stage 5-6) does the macronucleus undergo a 

 relatively rapid series of changes. At this time the nodes of the 

 nucleus begin to coalesce within the common nuclear membrane. 

 According to Johnson, this fusion occurs at separate loci because 

 it sometimes may result in a premature breaking of the chain. 

 Eventually the nucleus is compacted into one more or less spherical 

 mass in the center of the cell, though unsuccessful enucleation 

 experiments performed at this time indicate that occasionally one 

 or more nodes may remain isolated. Johnson described the 

 clumped nucleus as then showing a preliminary constriction 

 which lasts for about half an hour, then disappearing as the nucleus 

 elongates into a rod (Fig. 15B). But sometimes this constriction 

 was completed, the nucleus then and there separating into two 

 parts, not always equal. When unequal, the larger part showed a 

 secondary division later, resulting in a more equal allocation of 

 the nuclear material. These observations of exceptional behavior, 

 as well as the fact that division usually occurs later in the rod 



