| B43 
solution precipitates FrscHer’s crystalline salt (a potassium-cobalt 
sodium-nitrite). Macarrum puts sections of frozen material or teased 
out preparations of fresh tissue in his cobalt-nitrite-sodium-nitrite 
mixture. In the tissues and in the cells the potassium-precipitate 
can now be formed. After being thoroughly washed with distilled 
water, by which the reagent is washed away, the tissue is subjected 
to an after-treatment with ammonium sulphide, which renders FiscnEr’s 
sali black (formation of cobalt-sulphide). Wherever microscopical 
examination reveals black precipitates or black decoloration, we may 
decide upon the presence of potassium-compounds. 
If e.g. voluntary muscle-tissue is treated after Macuii.um’s method, 
it will be seen that the potassium-compounds occur almost exclusively 
in the doubly refracting discs of the muscular fibres. The discs 
of single refraction do not contain any potassium. Now, because the 
electric organs of the so-called electric fishes are developed (with a 
few exceptions) from voluntary muscular tissue, and since nobody 
has as yet studied the distribution of potassium-compounds in this 
metamorphosed muscular tissue, we considered an inquiry into the 
distribution of potassium compounds in the electric organs of some 
consequence. I regret to say that, although Dr. C. KerBerrt, Director 
of Natura Artis Magistra offered his kind assistance, the strong elec- 
tric fishes could not be put at my disposal. But also the Thorn-back 
(Raja clavata), occurring at the Dutch North Sea-coast, has an electric 
organ, which, though its action is weak, largely resembles in struc- 
ture the organs with stronger action. I, therefore, applied to the 
Zoological Station at Helder, where, through the kindness of Dr. 
Reprke and Dr. van Goor I was enabled to perform the potassium- 
reaction in the living electric organ of a thornback. The tissue was 
put in the reagent and afterwards made into sections with an ice- 
microtome (spray from liquid carbon dioxide). Although it is better 
to make the sections first and to treat them immediately after 
with the reagent, [ followed the other way, because there was no 
freezing microtome at the Station. This altered working-method, 
however, did not lessen the value of the results. I took care to put 
very small pieces of tissue in the reagent in order to allow the 
reagent to permeate the tissue as effectually and as rapidly as possible. 
As I do not see any difference between the outer and the central 
parts of the preparations, the diffusion appears to have succeeded 
well. Before reporting the results of this inquiry | may as well 
remind the reader that the electric organ of the Thorn-back is situ- 
ated within the. lower two-thirds of the tail. There it lies at the 
side of the dorsal and ventral tail-muscles. It consists of a large 
55* 
