AXIAL GRADIENT IX CILIATE INFUSORIA. 49 



If the susceptibility is in any degree a measure of the rate of 

 metabolism or more specifically of oxidation processes as it 

 unquestionably is in many other cases (Child 136) we must 

 conclude that an axial gradient in the rate of these processes is 

 a. characteristic feature of the ciliates. Moreover, the structure 

 and behavior of the animals also suggest differences in rate of 

 metabolic processes along the axis. In Stentor the peristome 

 region is undoubtedly the most sensitive, most highly contractile 

 and most active region of the body. In Vorticella and Car- 

 chesium a similar difference exists between peristome and body 

 exclusive of the contractile fiber of the stalk. In Stylonychia the 

 development and activity of cilia are greater in the anterior 

 region than elsewhere. In the case of Paramcecium, however, 

 the structure and behavior do not afford any definite evidence 

 upon this point. 



In forms like Stentor, Vorticella and Carchesium, where a special 

 contractile peristome region is present, this region is distinguished 

 from other regions of the body by a considerably greater degree 

 of susceptibility, while in Stylonychia the gradient of suscepti- 

 bility is almost uniform. Moreover in the elongated body of 

 Stentor the gradient is steeper in the anterior than in the posterior 

 half, i. e., in the region which forms the stalk the gradient is 

 relatively slight as compared with the body proper. In short 

 there can, I think, be no doubt that the differences in suscepti- 

 bility along the axis in Stentor, Stylonychia, Euplotes, Vorticella 

 and Carchesium are an expression of a dynamic axial gradient, 

 a gradient in the rate of certain fundamental metabolic processes. 



The case of Paramcecium requires some further consideration. 

 The constant association of the swelling with the vacuoles, and 

 in the great majority of cases with the anterior vacuole alone 

 raises the question whether we are to regard this region as 

 possessing a higher rate of metabolism or of respiration than other 

 parts of the body or whether some complicating factor, e. g., 

 weakness of the ectoplasmic wall in this region, plays a part in 

 the localization of the swelling. There is no visible indication 

 that the ectoplasm of the vacuolar region is thinner or structurally 

 different in any way from that of other regions of the body, but 

 such negative evidence is of course not conclusive. On the other 



