MORPHOGENESIS 



803 



gradient arises in Amoeba before a pseudopodium appears; it is greatest 

 at the tip of the developing pseudopodia, and greater in the more re- 

 cently produced than in the older ones. The bearing of axial gradients 

 upon the physiology of amoeboid movement is discussed at length in 

 Hyman's report. 



Apparently the differential susceptibility of various regions in P. 

 caudatum is not primarily dependent upon qualitative differences in the 

 protoplasmic constitution nor upon the precise mode of action of the 

 physical or chemical agent used to demonstrate it (Child and Deviney, 



a r\ 



Figure 184. Successive stages in the regulation of the mouth in a piece of Spirostomum 

 with the mouth at the anterior end. A, the operated individual; B, changes after one day — 

 the rounding off of the cut surface; C, twenty-four hours later — disappearance of the 

 original mouth and the development of a shallow peristomial depression on the side; 

 D, appearance of the new mouth four days after the injury. (From Seyd, 1936.) 



1926). For example ultra-violet radiation has a differential cytolytic 

 action, which is identical with that of chemical poisons (Child and 

 Deviney, 1926; Monod, 1933). It is believed that the environmental 

 agent alters in degree the physiological activities, with the differential 

 depending upon quantitative differences in the physiological condition. 

 Permeability and cytolytic gradients are therefore to be regarded as 

 manifestations, rather than causes, of an underlying physiological 

 gradient. 



The significance of physiological gradients in connection with the 

 regulation of specific organelles in Protozoa has yet to be made clear. 

 A more recent article devoted to the determinative action of the physio- 

 logical gradients is that of Seyd (1936) on Spirostomum amhtguum. A 



