204 THE BIOLOGY OF STENTOR 



Uhlig also regards the appearance of a primordium or new 

 membranellar band in connection with the locus of stripe width 

 contrast as expressing a circumferential gradient in propensity for 

 anlage formations, and this may be a fruitful way of regarding 

 these events. Certainly the granular stripe widths, or as he perhaps 

 more pertinently states, the distance between the fibrous clear 

 stripes, form an orderly gradient around the cell. Primordium 

 formation might therefore be regarded as always occurring at the 

 *' head end " of this gradient, or where the finest pigment stripes 

 are found. Whatever explanatory virtue the polarity or gradient 

 concept may have would then be applicable to happenings in this 

 region. Yet there are some difficulties which still need to be 

 resolved, for example, how primordium formations at transverse 

 sutures (see Fig. 460) can be regarded as expressing a gradient. 



A harmonious co-operation between the circular gradient 

 manifested in graded stripe widths and the polar gradient of mouth- 

 parts induction is, according to Uhlig, necessary for complete oral 

 development. The former guides the location and longitudinal 

 development of the membranellar band, later invagination of its 

 posterior end to form the mouthparts being induced by some 

 influence having its high point near the posterior end of the cell. 



When a stentor divides or is cut in two, there would be, in 

 Uhlig's conception, readjustment to a new equilibrium in which 

 the original single polar gradient is converted into two. As inti- 

 mated above, short tail-pole fragments may therefore be slow in 

 regenerating because of their need for greater readjustments before 

 significant polar differences can be re-established. When stentors 

 are cut in two and rotated so that anterior and posterior stripe 

 systems cannot rejoin, a conflict between double but homopolar 

 gradients apparently ensues, which is resolved in various ways to 

 be described later (p. 227). 



Stentors therefore may be said to bear within the structure of 

 every part of the cortex an antero-posterior and a left-right 

 polarization. In addition, there is experimental evidence for polar 

 and circular gradients of paramount importance in the elaboration 

 of major ectoplasmic organelles. 



