336 Proceedings of the Koyal Society of Edinburgh. [Sess. 
results were got after the lapse of from twenty-four to' 
thirty-six hours. 
The tissues must be kept moist and in a cool atmosphere during the 
period allowed for post-mortem change. In the case of the tissues of adult 
animals they were generally left in situ until the necessary lapse of time 
had occurred. 
The silver-nitrate method stains fully developed nerves a dark brown, 
almost black colour, the rest of the tissues being a golden yellow or light 
brown. As a general rule the nerve fibres stain better than the nerve 
cells. With embryonic nerve tissue the silver nitrate shows a 
curious selective action, in that it stains the developing nerves 
in accordance with their degree of development. The more 
advanced the development of a nerve is, the better is it stained. This pro- 
perty of the stain made it singularly well adapted for the investigation of 
nerve development. 
Since the silver nitrate stains other tissues as well as nerves, some 
difficulty is at times met with in distinguishing nervous from non-nervous 
tissue. Several factors, such as the microscopic appearances, the relative 
size and thickness, and the position of the structures, serve as guides ; but 
a peculiar sheen, recognised after a little practice, and belonging solely 
to nerve tissue stained by silver nitrate, forms the best distinguishing 
guide. Unfortunately, the stain is not always permanent ; many sections 
show after the lapse of about two years either fading or granularity. On 
the whole, embryonic tissue seems to retain the stain better than adult. 
Some attempts have been made to fix the stain with hyposulphate of soda r 
but the success of this plan is as yet doubtful. 
2. Material. 
Embryonic chicks at stages varying from two to seven days’ incubation 
have formed the material upon which this investigation has been carried 
out. 
The accounts given by different workers of the successful utilisation of 
embryonic fish in similar investigations led to experiments being made 
with this material. Plaice eggs were procured from the Bay of Nigg 
Hatchery, Aberdeen, through the courtesy of Dr Williamson, but, owing to 
technical difficulties connected with the use of the silver-nitrate stain, the 
results were disappointing. It was found that immersion of the whole egg 
in the silver solution was followed by such a degree of hardening that 
the egg chipped under the microtomic razor. Attempts were made to 
