64 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 



polyp, not, however, from either of the original ends, but from the 

 central portion of the piece which is farthest from the glass bottom 

 of the vessel and most freely exposed to the water and therefore to 

 oxygen.^ In this case, an original polarity-gradient has not merely 



H- 



g^^^^^^ 



Fig. 25 

 Control of polarity by external agencies in Hydroids. A series of internodes of 

 Obelia regenerating towards the anode when exposed to the passage of a weak 

 electric current. The control series at the same stage had all regenerated hydranths 

 at both cut ends. (From 'Lund, Jour n. Exp. Zool. xxxiv, 1921.) 



been reversed, but the original polarity has been obliterated, and 

 a wholly new polarity induced (fig. 26). 



The winter-buds of the social Ascidian Clavellina appear to be 

 irregular aggregations of cells with no relation to the polarity of 

 their parent. The polarity of the zooids which later arise from these 



^ Child, 1925 B, 1927. 



