DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 37 



the body, and apparently in some cases in the nervous system. 

 Even in such a case, however, some degree of nervous control 

 by more anterior over more posterior regions is possible because 

 nervous differentiation with its peculiar possibilities of rapid 

 and intense excitation and transmission progresses posteriorly. 

 In other words, the increasing excitabihty and conductivity, 

 with increasing differentiation of the nervous system, makes 

 it possible for a segment or a group of segments to control to 

 some extent more posterior, physiologically younger segments 

 with a higher intrinsic metabolic rate, especially when the excita- 

 tion is intense, for a sufficiently intense excitation may proceed 

 for a considerable distance up the developmental gradient in 

 the posterior region. Observation of the behavior of many of 

 the elongated oligochetes suggests that the posterior body- 

 region, in which the gradient rises toward the posterior end is 

 not completely subordinated to more anterior regions, except 

 in relatively intense excitation. As regards the polychetes, the 

 behavior and the degree of segmental coordination indicates that 

 the degree of segmental integration is greater in at least many 

 forms than in the oligochetes referred to. Probably a study of 

 the susceptibility gradients in the adult polychetes will show a 

 shorter or in forms with a definite number of segments no 

 posterior region in which the secondary developmental gradient 

 persists. 



According to this interpretation, the gradient which first 

 appears in the segmental region, a gradient decreasing in rate 

 from the growing region anteriorly is an intrinsic developmental 

 gradient of secondary origin, for instance, it is an indicator 

 of the physiological age and stage of differentiation of the suc- 

 cessive segments. The later gradient, decreasing in rate from 

 the first body-segment posteriorly, is a gradient of extrinsic 

 stimulation, resulting from the physiological integration of the 

 segments through the development of functional nervous control. 

 It may be called the integrative gradient. Since the head region 

 is, to a greater or less extent, the dominant region in such func- 

 tional nervous control, this later and persistent gradient is in 

 the same direction as the primary apico-basal gradient and rep- 



