56 LIBBIE H. HYMAN 



These gradients are demonstrable by several direct and in- 

 direct methods. The chief direct method consists in the exposure 

 of the organism to solutions of certain substances of sufficient 

 concentration to kill it within a few hours; the differences in 

 time of death of the various levels of the body are then observed. 

 A number of reagents of widely different chemical composition 

 have been used for this purpose — potassium cyanide, several sub- 

 stances belonging to the class of narcotics, acids, alkalies, some 

 salts, and one or two vital dyes — and all of them yield essentially 

 the same death gradients. When colored substances are em- 

 ployed a beautiful color gradient may often be demonstrated in 

 the organism before its death, the most susceptible regions be- 

 coming more deeply colored. Thus the mixture of dimethyl- 

 p-phenylene-diamine and a-naphthol yield within the animal a 

 blue precipitate of indophenol, the amount of precipitate show- 

 ing a definite and well-marked gradation along the antero- 

 posterior axis. Gradients in organisms can also be demonstrated 

 by means of a sensitive galvanometer, although the method has 

 never been employed for this purpose intentionally. Mathews 

 ('03) records that the head of coelenterate polyps is electro- 

 negative* to the stem, and anterior levels of the stem negative 

 to posterior levels; and Child ('14) later showed that in these 

 forms anterior levels are always more susceptible to the reagents 

 mentioned than posterior levels. Morgan and Dimon ('04) 

 repeated Mathews' experiment, using the earthworm, and found 

 that the anterior and posterior ends of this animal are electro- 

 negative to the middle, and I demonstrated that in annelids in 

 general the anterior and posterior ends are more susceptible to 

 cyanide than the middle regions (Hyman, '16 a). In this con- 

 nection it is worthy of remark that most Protozoa when sub- 

 jected to an electric current orient themselves so that their an- 

 terior ends are directed toward the cathode and swim to the 



3 Electro-negative with regard to the external current; in reality anterior 

 levels are electro-positive to posterior ones. It is very unfortunate that it has 

 become customary to refer to the bioelectric currents according to the direction 

 in which the current passes through the galvanometer and not in which it passes 

 through the tissue. 



