METAPLASM; POLARITY 139 



due to different environments in the tissue. For these workers 

 this physiological differentiation is the essential element of polarity; 

 any morphological polarity is due secondarily to it. 



Metabolic Gradient. The most suggestive physiological conception 

 recently developed in this connection is that of Child (1911-1916). 

 Child has shown in the case of Planaria and other lower animals, as well 

 as in certain algae, that along each of the axes of symmetry there exists a 

 " metabolic gradient," or "axial gradient:" the rate of the physiological 

 processes is highest at one end of the axis and diminishes progressively 

 toward the other end. The anterior end of a planarian, for example, 

 has a higher metabolic rate than the posterior portions. Furthermore, 

 the portions of higher rate dominate and control the development of 

 those portions having a lower rate, with the result that the young indivi- 

 dual soon develops and maintains a definite physiological correlation of 

 anterior and posterior parts. Similarly in individuals with more than one 

 axis of symmetry, there may be a corresponding dorsal- ventral, as well 

 as an axial-marginal, correlation. That polarity is here primarily a 

 physiological matter is indicated by the fact that experimental altera- 

 tions in the metabolic rate in different parts is followed by abnormalities 

 in structural development. 



As to the means by which the dominance of certain regions over others 

 is exercised, correlating the activities of the various parts of the or- 

 ganism, there are two principal theories in the field. According to one 

 theory chemical substances (hormones) are produced at certain places 

 and transmitted through the body. Although the circulation of such 

 hormones clearly has much to do with correlation in higher complex 

 organisms, Child adduces good evidence in support of the second theory, 

 namely, that the fundamental relations of polarity " depend primarily 

 upon impulses or changes of some sort transmitted from the dominant 

 region, rather than upon the transportation of chemical substances" 

 (p. 224). 



It cannot at present be said to what extent this conception of polarity 

 is applicable to the single cell. The work of Child shows in a very 

 definite manner the coincidence of the morphological and physiological 

 axes of polarity, which indicates that the two are but different aspects of 

 one and the same polar differentiation. A similar coincidence exists very 

 generally in the case of the single cell. In the cell, as in the organism as a 

 whole, functional and structural differentiation are inseparably connected. 

 In the present state of our knowledge the attempt to determine the real 

 essence of polarity raises questions which cannot yet be answered. Does 

 physiological polarity depend upon a polarized structure which is a 

 fundamental attribute of the cell's ultimate organization? Or does a 

 polarized morphological arrangement follow and depend upon a physio- 

 logical division of labor arising as a difference in intensity or rate in proc- 



