DOMINANCE AND PHYSIOLOGICAL ISOLATION 327 



region in front of the anal segment in annelids suggests a partial physio- 

 logical isolation from anterior dominance at this stage, and the repetitive 

 formation of segments appears to be a reproduction resulting from some 

 degree of physiological isolation of each segment primordium from the 

 slight and short-range dominance of the growing region. The segment re- 

 mains a segment, instead of becoming a complete individual, either be- 

 cause it is subordinated as it develops to anterior dominance, probably 

 through the nervous system, or because capacity for head development 

 is absent. In some annelids subordination to anterior dominance is evi- 

 dently the factor maintaining segmental character, for, when more an- 

 terior parts are removed, a head regenerates from the segment or segments 

 adjoining the level of section. In other species the segment remains es- 

 sentially unaltered when more anterior parts are removed, but it is still 

 able to react to removal of posterior parts by regeneration of a posterior 

 end and a new segment-forming region. Successive physiological isola- 

 tions in the mesoderm may be involved in segment formation in other 

 segmented animals. Probably physiological isolation, perhaps local in ec- 

 toderm, mesoderm, or entoderm, plays a part in determining localization 

 and order of various other reduplications of organ systems and parts. 

 Such isolations may be temporary, occurring at certain stages of develop- 

 ment and being followed by reintegration. 



DOMINANCE AND PHYSIOLOGICAL ISOLATION IN THE FUNCTIONAL 

 ACTIVITIES OF CERTAIN ORGANS 



The ctenophore plate row affords an extremely interesting example of 

 functional dominance and physiological isolation. '•^ Each row is a gra- 

 dient with high end toward the apical (aboral) pole of the body, but each 

 plate in a row is capable of independent movement. Ordinarily, the whole 

 row is dominated by rhythmic impulses transmitted through the nervous 

 system from the central nervous tissue about the apical pole. When the 

 whole animal is subjected to anesthetics, cyanide, or various other depres- 

 sing agents in gradually toxic concentrations, the impulses from the apical 

 nervous system become less frequent and perhaps weaker, and in the 

 plate row susceptibility to the toxic agent decreases from the apical to 

 the oral end. After some time under the depressing conditions a new 

 rhythmic impulse, independent of, and more rapid than, the depressed 

 apical impulse, begins to appear in the less susceptible and less inhibited 

 oral two-thirds, half, or less of the row, according to experimental condi- 



'•3 Parker, 1905; Child, 1917c, 1921a, pp. 212-20, 1933a, and literature cited in these papers. 



