10 E. J. LUND 



2. Reversal of polarity in normal individuals 



In plate 5 are shown individuals of varying degrees of abnor- 

 mality. Figures 27 and 28 are individuals nearly normal except 

 for the spine. They had been in tap water without food, and 

 were perfectly normal when isolated eighteen hours previously. 

 Figures 29 and 30 were drawn eighteen and twenty-four hours 

 respectively after isolation of perfectly normal individuals in tap 

 water. The larger individual appears to be 'dominant.' Cilia- 

 tion is unipolar except for cilia in the oral groove of the smaller 

 end, which were beating backward Figure 31 is a heteromor- 

 phic individual resulting from a normal individual after being 

 isolated in weak solution of Horlick's malted milk. Figure 32 

 is the same individual drawn two hours later. The histories of 

 the individuals shown in plate 5 were not followed further. 

 They suffice however to show that reversal of polarity may arise 

 without cutting and that different degrees of this tendency to 

 form heteromorphic individuals occur. 



V. POLARITY, DOMINANCE AND RESISTANCE OF STRUCTURES TO 

 DEDIFFERENTIATION 



Whether dedifferentiation in Bursaria can lead to disappearance 

 of polarity in the cell remains an open question, for although at 

 the stage of the ciliated sphere which is shown in figure 7, there 

 appears to be no visible anterior or posterior, neither in structure 

 nor in swimming movements, nevertheless the protoplasm in the 

 cyst has a polar axis (fig. 8). 



The protoplasm of a normal posterior end may reverse its po- 

 larity and by dedifferentiation form a partial or complete ante- 

 rior end (figs. 29, 30, 31, and 32). Similarly two anterior ends 

 may arise simultaneously from the same dedifferentiated 

 piece (figs. 21 a', 21 b). Heteropolarity can appear in ciliary 

 action without any very marked morphological evidence of 

 heteromorphic condition (fig. 22). Compare figures 29 and 30. 



In every heteromorphic pair with members of unequal size, the 

 smaller and weaker member sooner or later dedifferentiated and 

 the whole or part of its substance became part of the stronger 



