Houser, The Neurones of a Selachian. 75 
cell-bodies are usually bipolar in form; (2) that there is but 
slight demarcation between the cell-body and its protoplasmic 
processes ; and (3) that the chromophile substance is less differ- 
entiated than in teJeosts and higher vertebrates. 
Section III. Mertsonps oF INVESTIGATION. 
The technique employed in the course of this research has 
covered a wide range. In fact, every process holding any prom- 
ise of value has been given a careful trial. It seems desirable 
to describe here, however, only those methods which have con- 
tributed most largely to the final results. 
Z.  Chrome-Silver Inpregnation. 
The production of a chrome-silver deposit in nervous ele- 
ments is far more than a simple chemical reaction between the 
potassium bichromate and the silver nitrate employed. There 
is to be added as a prime factor, the chemistry of the nervous 
tissues themselves. The substances present in the nervous ele- 
ments enter into the reaction to such a degree that the results 
are either positivé or negative according to the character of 
those substances. In the economy of the animal, serving both 
as the basis for nervous activity and produced as the result of 
it, there is a constant round of metabolic change altering the 
chemical composition of the nervous tissues. In my work 
upon Mustelus, it was soon found that strict account had to be 
taken of the physiological state of the animal. An individual 
fresh from the pursuit of his prey in the open sea gave <alto- 
gether different results from one which had been kept for some 
time ina small aqarium, simply because the reactions of the 
tissues in the two instances were quite different. And so, be- 
fore even fair results with chrome-silver impregnation could be 
secured, it was necessary to make a careful study of physiolog- 
ical conditions. 
It was also found that selachian nervous elements lend 
themselves but grudgingly, at best, to the reaction desired. 
Numerous trials were made of the several published schemes 
for securing: impregnation, but particularly of the procedure 
