DOMINANCE AND PHYSIOLOGICAL ISOLATION 325 



ceding division, and the location of the fission plane is apparently deter- 

 mined soon after the preceding fission and several hours before it be- 

 comes visible (Peebles, 1912). Under certain experimental conditions Par- 

 amecium forms chains of zooids (Hinrichs, 1927), and some protozoan 

 species form chains naturally. Some of the cihates that form multiaxiate 

 complexes give rather definite evidence of dominance and physiological 

 isolation in the localization and sequence of new zooids — for example, 

 Zoothamnium (Faure-Fremiet, 1930; Summers, 19380, h). Some of the 

 remarkable forms of agamic reproduction among the Suctoria present ex- 

 tremely interesting problems for the future; at present the determining 

 conditions are quite unknown (see pp. 609 14). In other suctorian species 

 forms of budding apparently essentially similar to those in other groups 

 appear. 



Budding sequences in bryozoa afford beautiful examples of definite 

 spatial patterns, indicating dominance and physiological isolation, and 

 reconstitution also indicates dominance in axial relations.'" Development 

 of buds from stolons in sessile ascidians suggests that physiological isola- 

 tion is a factor in their origin. The remarkable types of budding in the 

 pelagic tunicates — for example, the migration of cell groups from the 

 ventral to the dorsal stolon and the difference between median and lateral 

 buds on the dorsal stolon of Doliolum, and the periodic arrangement of 

 buds in blocks or "wheels" in certain of the Salpidae — also suggest exist- 

 ence of very definite spatial patterns of ordering and control of these 

 phenomena; but here, too, experimental data are lacking." The relation 

 between agamic reproduction and regression of the original individual, 

 as described by Berrill (1935) for various ascidian species, indicates that 

 physiological isolation by decrease or elimination of dominance is con- 

 cerned. 



AUTOTOMY AND FRAGMENTATION 



The triclad Fonticola velata kept at or above 20° C. with abundant 

 food, on attaining a certain length, ceases to feed, the pharynx degen- 

 erates, and the body undergoes repeated fragmentation from the posterior 

 end anteriorly. Fragmentation at each successive level is preceded by in- 

 ternal degenerative changes. The isolated fragments, usually less than a 

 millimeter in diameter, encyst and, after extensive degeneration and re- 



■° See, e.g., Brien, 1936, and his citations of earlier work; Brien et Huysmans, 1937, for pat- 

 terns of budding; Otto, 192 1, for axial relations in reconstitution. 



" In this connection see M. E. Johnson, 1910; Ritter and Johnson, 191 1. 



