DOMINANCE AND PHYSIOLOGICAL ISOLATION 305 



that at a certain variable distance dominance becomes ineffective as a 

 factor determining development or maintenance of parts already devel- 

 oped. In protoplasms lacking specialized conducting paths the effects of 

 local excitation are apparently transmitted with a decrement; and even 

 in cases in which some part of the central nervous system appears to 

 be the dominant region, evidences of limited range of dominance often 

 appear in the simpler animals; but whether this limitation results from an 

 actual decrement in nervous transmission in primitive nerve complexes 

 or from other factors — for example, incomplete differentiation of the nerv- 

 ous system in regions of rapid growth in length — is uncertain. Protoplas- 

 mic excitation and transmission attain their highest development in nerv- 

 ous tissue. The early development of the nervous system, the localization 

 of the chief masses of nervous tissue in, or in close association with, the 

 high regions of gradients, and the close parallelism between relations of 

 dominance and subordination within the nervous system and gradients 

 of earlier stages suggest that the nervous system is a relatively direct de- 

 velopmental expression of the primary factors in organismic integration. 



A part of an individual which, for any reason, comes to lie beyond the 

 range of effective dominance is physiologically isolated, that is, it is no 

 longer subjected to the factors which were concerned in determining its 

 development as a part of the individual or its persistence as a particular 

 part. If the part is not so stably determined or differentiated that it 

 cannot react to this isolation, it tends to lose more or less completely its 

 characteristics as a definite part of the individual and may, under certain 

 conditions, reconstitute a new individual. 



This limitation in the effective range of dominance is a factor in limiting 

 length of the individual or zooid in many organisms, but the limit of domi- 

 nance and of length varies with conditions. An intensely active dominant 

 region determines, in general, a greater length than one less active — for 

 example, in various flatworms. Also, a greater length of individual or 

 zooid is attained in planarians, Stenostomum, and various other forms with 

 slow than with rapid growth, probably because differentiation of the 

 longitudinal nerve cords in the regions of most rapid growth more nearly 

 keeps pace with a slow, than with a rapid, increase in body length. In 

 planarians and Stenostomum the length of a single zooid increases with 

 advance in development of the head. If growth in length of a Stenostomum 

 chain is rapid, a new fission zone and head region arise at a shorter dis- 

 tance from a zooid head in early developmental stage than from a fully 

 developed head. 



