330 PATTERNS AND PROBLEMS OF DEVELOPMENT 



or experimental conditions subordinate regions may become dominant, 

 in consequence either of decrease in the pre-existing dominance or of irri- 

 tation or direct stimulation of a subordinate region, and the direction of 

 dominance may be reversed locally or over a considerable length of the 

 tract. A more or less complete physiological closure of the morpho- 

 logically completely open intestine may result from local reversal in 

 direction of dominance and contraction produced by a local irritation 

 or lesion. The regions normally dominant represent the high ends of gra- 

 dients, as indicated most distinctly in the small intestine, by graded dif- 

 ferentials in rate of rhythmic contraction in isolated pieces, rate of res- 

 piration, length of latent period, irritabihty, tone, etc.'-* Here, again, the 

 similarity to the ctenophore plate row and to the heart is evident, as Alvarez 

 1928, chap, vii) has pointed out. A similar functional dominance is ap- 

 parently present in the ureter and will probably be found in various other 

 elongated organs in which functional activity progresses in a definite direc- 

 tion. 



CONCLUSION 



The evidence from agamic reproduction in both plants and animals and 

 from reconstitution of isolated pieces indicates that physiological domi- 

 nance tends to maintain the individual, zooid, axis, or part as an inte- 

 grated unit, while physiological isolation, like physical isolation, antag- 

 onizes this unity and paves the way for origin of new individuals, zooids, 

 axes, or parts. For the new development of the physiologically or physi- 

 cally isolated part, however, a new dominance and establishment of a 

 new gradient or gradients are necessary; and these changes are possible 

 only when the isolated part, or some of its cells, are capable of reacting 

 to the isolation by activation and attainment of a more generalized be- 

 havior and so of initiating the reorganization. 



The importance of the nervous system in dominating and integrating 

 activities in later development and mature life is a familiar fact. Its early 

 differentiation and the localization of the chief aggregations of nervous 

 tissue in the high regions of the gradient pattern present in early stages 

 suggest that, as regards the general features of its pattern, the central 

 nervous system represents the morphological expression of the higher gra- 

 dient-levels and is physiologically a further development of the primitive 

 type of dominance. The influence of the nervous system in maintaining 



'4 Most of our knowledge of these relations in the alimentary tract is due to Alvarez and 

 his co-workers. See Alvarez, 1928, and papers cited there. 



