CHAPTER XI 



POLARITY 



Polarization as a graded " difference " is probably a precondition 

 for the achievement of persisting organic form, for we cannot 

 imagine how potentiahties for development could begin in an 

 entirely anisotropic system. Although specific form is not explain- 

 able in terms of polarities, which are almost universal in the organic 

 world, the guidance of development is traceable thereto; and 

 polarity may well be intimately involved in the first stages of 

 differentiation itself (see Bonner, 1958). For radially symmetric 

 organisms like Acetabidaria, cellular slime molds, and higher 

 plants, antero-posterior polarization may be sufficient, but Stentor 

 is asymmetric in the position of its mouthparts and the pattern of 

 the lateral striping. In the ciliate we may therefore expect to find 

 transverse as well as axial gradients in some intimate property of 

 the cortical cytoplasm ; and in addition, the structural elements of 

 the ectoplasm, which persist in fragments and in whole cells in 

 reorganization and division, have a built-in polarity and asymmetry. 

 The importance of polar differences in explaining form has been 

 emphasized above all by Child (1941). Consciously or not, the 

 tradition he established has continually been drawn upon. Child 

 also included Stentor in the scope of his investigations, yet his 

 findings in this context are here considered in a different chapter 

 because they seem, for the most part, to be more indicative of 

 structural differences. In the autonomous differentiation of stentors 

 there is better evidence for gradients than in responses to external 

 agents. 



I. Fixity of structural polarity 



There is abundant evidence that structural polarization charac- 

 terizes the formed components of the ectoplasm. As already 

 described, ciliary row s or kinetics are intrinsically polarized because 

 they follow the general rule of desmodexy in ciliates : the fiber or 



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