66 THE BIOLOGY OF STENTOR 



the appearance is that the left boundary stripe is branching. 

 Schuberg (1890) therefore called this area the "ramifying zone", 

 evidently the area of stripe multiplication and also the site of the 

 oral primordium. In the left anterior corner of this triangle the 

 widest pigment stripes begin splitting into narrow stripes. As 

 the split proceeds posteriorly the next wide stripe to the left begins 

 splitting, wdth the result that a series of stripes of ever increasing 

 length is formed to the right and the characteristic ramifying zone 

 is thus achieved. At least this is the general impression, though 

 other details doubtless need to be added. If so, growth takes a 

 spiral course, as it were, with the zone of increase gradually moving 

 to the animal's left as new short stripes are added and older stripes 

 to the right increase in length until they reach the posterior pole. 

 Correspondingly, the oral primordium which appears in this zone 

 would continually shift leftward, with the result that new mouth 

 parts appear always somewhat to the left of those preceding. Such 

 spiral growth recalls that of the fruiting body in certain fungi 

 (Delbriick and Reichardt, 1956). 



We have already remarked that pigment stripes are mere fill-ins 

 and their splitting is doubtless due to the emergence within them 

 of new clear stripes with their ciliary rows and fibrous structures. 

 Each new clear stripe would then not be connected with others, 

 corresponding to the description of Villeneuve-Brachon (1940). 

 Later the clear stripes do join together and cut off the split 

 branches of a pigment band. Older figures thus show fibrous struc- 

 tures of the clear stripes as branching and re-branching in the 

 ramifying zone. From what we now know about kinetics and 

 myonemes it is evident that anastomosis would entail great struc- 

 tural difficulties and we have to leave this problem until appro- 

 priate EM studies are available. Because the number of stripes 

 tends toward a fixed upper limit, the need for stripe multiplication 

 may therefore vary, possibly being minimal in stentors that have 

 lived for a long time without dividing. This would account for the 

 observations of Johnson, and much later of Dierks, that the 

 ramifying zone is variable in its aspect, sometimes even un- 

 identifiable as such. 



Multiplication of clear stripes could not occur by simple 

 splitting since this would leave one branch with cilia and one 

 without. One branch would have to migrate sub-cortically and 



