48 THE ORIGIN OF THE NERVOUS SYSTEM 



and in the " hairs " of algae (Child, 1916^, e, 191 ja, 

 iqige), in the larger, slow moving flagellum of Noctiluca, 

 in the tentacles of Hydrozoa (Child and Hyman, 1919; 

 Child 19196) , in the plate rows of ctenophores (Child, 

 191 yc), the growing arms of echinoderm larvae, the 

 branchiae and sensory tentacles of various annelids, 

 the growing tail of the ascidian and the amphibian tad- 

 pole, etc. Dr. Hyman has found that the embryonic 

 heart of the chick and of the fish represents a suscepti- 

 bility gradient with the high region at the sinus end. 

 But the most extensive work on the metabolic gradient 

 of any organ is that of Alvarez and his assistants on the 

 vertebrate alimentary tract. 1 They have found in the 

 small intestine a gradient in irritability, latent period, tone 

 rhythm, conduction, susceptibility to various drugs, and 

 gradients in at least some of these conditions in the wall 

 of the stomach and colon. Tashiro (1917 and earlier 

 papers) has found a gradient in CO 2 production in cer- 

 tain nerves, the direction of functional conduction being 

 down the gradient and in certain of these nerves a 

 susceptibility gradient has been observed (Child, 19140). 

 In an investigation of the respiration of ground 

 nervous tissue C. G. MacArthur and Jones (1917) have 

 found differences in rate of respiration in different parts 

 of the central nervous system which indicate the existence 

 of a gradient in rate of respiration. The rate of respira- 

 tion is highest in the cerebrum and decreases in the 

 various parts in the following order: cerebellum, mid- 

 brain, medulla, corpus callosum, spinal cord nerve. The 

 authors find that gray matter consumes about twice as 



1 Alvarez, 1914, 19150, b, 19160, b, 19170, b, 19180, b, c; Alvarez 

 and Starkweather, 19180, b, c, d, e, 1919; Alvarez and Taylor, 19170, b. 



