84 PLATYHELMINTHES 



organism. Dr. Child kindly consented to prepare for this text a 

 brief summary, which we are privileged to print without alteration. 



"Many different lines of evidence, observational and experimental, 

 indicate that physiological polarity of axiation in general is in its simplest 

 terms a gradation or gradient in physiological condition along the axis in 

 question involving both quantitative differences in metabolism and proto- 

 plasmic constitution. During development the primary gradient or 

 gradients may be altered, may disappear and new gradients may arise so 

 that the original gradients do not necessarily persist in the adult organism. 

 Localization and differentiation at different levels of an axis result from 

 the differences in physiological condition at different levels of a gradient 

 and are often factors in altering or obliterating the original gradient. 



"The evidence also indicates that such gradients arise or originate 

 in the reaction of a cell or a cell mass to some environmental differential, 

 but after its establishment a gradient may persist through cell division or 

 other reproductive process and so be inherited by the offspring of such 

 reproduction. A gradient may be determined by the localization, 

 experimentally or otherwise, of a region of increased physiological activity 

 in a cell or cell mass. The gradient in its beginning may be nothing more 

 than the gradation in activity from the center to the periphery of such a 

 region. In consequence of growth the center of such a region may be- 

 come an apical or anterior end of an axis. The environmental differen- 

 tials which determine gradients may be of various sorts, light, electric 

 current, local stimulation, differential exposure to oxygen, etc., and in the 

 case of organ axes, buds, etc., the environmental factors determining the 

 gradient may consist in the relations of the part concerned to other parts 

 of the organism. 



"The high or most active end of a gradient may exercise a physio- 

 logical dominance over other regions. This dominance in its more 

 primitive form apparently decreases in effectiveness with increasing 

 distance from the dominant region and if a part of the organism comes, 

 either through increase in size of the organism or through decrease in 

 dominance or certain other conditions, to lie beyond the range of dom- 

 inance, physiological isolation of the part results and in many of the 

 simpler organisms such physiological isolation may result in agamic 

 reproduction. 



"The gradient theory has no quarrel with heredity. The gradient 

 merely provides the plan, the pattern, the framework, so to speak, while 

 the material and its possibilities are given in the hereditary constitution 

 of the protoplasm in which the gradient exists." 



