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Proceedings of the Royal Society of Edinburgh. [Sess. 
argued that the rhythmical action and refractory period must depend on 
Auerbach’s plexus. The question as to where the Auerbach plexus received 
the stimuli necessary for the causation of the pendulum movements was 
discussed. 
In this connection the work of Roger (24) and Straschesko (25) was 
referred to. They suggested that the contents of the intestine might 
furnish the necessary stimuli. The fact that the pendulum movements 
persisted after separation of the longitudinal from the circular coats 
excluded this theory. Magnus suggested that the stimuli might be afforded 
by a material previously developed in the nerve centres of the plexus. A 
considerable portion of the paper dealt with “ acting point ” of poisons in 
the gut. 
Present Investigation. 
1. Methods. 
In this investigation the silver-nitrate staining method of Ramon y 
Cajal was used. The method consists in impregnating the tissue with the 
silver salt, and subsequently exposing it to the action of a reducing agent. 
Three modifications of this method have been described by Ramon y 
Cajal (27). From experience with the method and its various modifications 
it was found that the original method gave the most satisfactory results 
with chicks at the very early stages of development. The first modification 
(27a) proved most successful with chicks from four days of incubation and 
onwards. The second modification (27a) also gave very good results 
with chicks after four days of incubation, but seemed on the whole more 
adapted for adult tissue. The third modification (27b) was introduced by 
Ramon y Cajal for work on the lumbricus, and with this material it certainly 
gives most beautiful results, but with embryonic tissue it was not found 
to be a success. 
For some time the uncertainty of the results of this staining method 
gave rise to considerable trouble. Great care was taken with the technique, 
but nevertheless the number of unaccountable failures remained high. A 
number of small experiments were made, among which was that of em- 
bedding the embryo in a thin coating of agar previous to immersion in the 
silver solution. This was tried in the hope of preventing the deposition 
of the silver round the embryo, an accident which frequently occurred with 
embryos of two to three days’ incubation. The embedding in agar was 
found to prevent this, and to in no way hinder the penetration of the silver 
salt. With chicks of more than three days’ incubation the agar was found 
