ioo THE ORIGIN OF THE NERVOUS SYSTEM 



transportation of formative or inhibiting substances, 

 but even in these plants it can be blocked by a zone of 

 low temperature about the stem between dominant and 

 subordinate parts, which merely decreases the metabolic 

 activity of the cooled region but does not stop trans- 

 portation (Child and Bellamy, 1919). Moreover, the 

 relation of dominance and subordination exists in algae 

 in which a transportative mechanism is not, so far as is 

 known, developed. 



A similar relation exists in animals, the apical or 

 anterior region being primarily the dominant region in 

 the polar axis, and it has been possible to show experi- 

 mentally that this relation plays an important part in 

 the localization, proportions, and relations of parts. 1 

 In Planaria, for example, a piece taken from any level 

 of the body is capable of giving rise to all parts posterior 

 to its own level, even if it does not give rise to a head, 

 but it never produces any part anterior to its own level 

 unless some degree of head development occurs first 

 (Figs. 8-12). Moreover, the distance from the head at 

 which certain organs arise in regeneration can be altered 

 experimentally by altering the rate of physiological 

 activity in the regenerating head region (Child, 191 ic). 

 In short, any given level of the body dominates more 

 posterior levels to a certain degree, and is dominated by 

 more anterior levels. 



Although the relation of physiological dominance 

 and subordination originates in the physiological gradient, 



1 In 1907 I called attention to the existence of such a relation in the 

 hydroid, Tubularia (Child, 19070, c, 19126), and Rand's work on Hydra 

 led him to recognize its existence in that form (Rand, 1911, 1912). 

 Since then further data bearing on the question of dominance and its 

 relation to physiological condition have accumulated (Child, 191 ic, 

 1914^, e, 1915^, chap, iv, 19166). 



