THE REFLEX ARC 255 



in press, and in the frog by Bellamy (1919). As the 

 body develops, this region moves posteriorly and repre- 

 sents a posterior growing region of the embryo. Hymah 

 has found that in the fish its susceptibility is equal to, 

 or higher than, that of the head and much higher than 

 that of more anterior postcephalic levels. Moreover, 

 it is electronegative externally to all levels of the body 

 behind the head. Bellamy has found that the suscep- 

 tibility relations of this region in the frog are essentially 

 similar to those in the fish, and the newly hatched tad- 

 pole shows a double gradient in susceptibility and 

 electric potential, both susceptibility and external elec- 

 tronegativity decreasing from the posterior tail region to 

 the body and from the head to the posterior body 

 region, the posterior tail region showing at this time 

 the highest electronegativity of any part of the body 

 (unpublished) . The reaction of the tadpole in a constant 

 current, which consists in orientation with head toward 

 the anode, instead of toward the cathode, as in most of 

 the lower animals, is unquestionably dependent upon 

 these physiological conditions. In still later stages of 

 vertebrate development the posterior growing region 

 disappears and with it usually the secondary gradient, 

 though in fishes and in the tailed Amphibia it may 

 persist to some extent in the tail region. 



These facts show that there is a period of develop- 

 ment in the vertebrates during which the longitudinal 

 physiological gradient in the postcephalic region has its 

 high end posteriorly and its low end anteriorly, i.e., 

 just behind the head. Moreover, the presence of elec- 

 trical differences associated with this gradient has been 

 demonstrated in fishes and Amphibia. As regards the 



