82 Miss M. P. FitzGerald. The Origin of the [June 4, 
indications of these, the cytoplasm was, as a rule, free from colour, but some- 
times, under the most favourable daylight illumination, it appeared to be tinged 
with a faint, almost imperceptible, blue. In some cases it was uncertain 
whether this was due to a scattering of the Hight reflected from the material 
in the canaliculi. 
In view of the fact previously stated (p. 69), that the presence of a free 
acid is a necessary factor for the production of Prussian blue im vitro, from 
the interaction of solutions of ammonium ferric citrate and potassium ferro- 
cyanide, the occurrence of the Prussian blue reaction in the canaliculi of 
the parietal cells of an animal injected with a solution of these two salts affords 
conclusive evidence of the presence of free acid within these structures. 
The evidence of the occurrence of the reaction was obtained in @ very 
limited portion of the mucosa. The examination of a large number of 
sections cut from the same part of the mucosa as those which furnished 
the evidence of Prussian blue within the parietal cells and the lumina of the 
tubules afforded no proof whatever of the presence of the blue compound in 
the glandular structures, | 
This indicates the regional nature of the secretory activity, and suggests 
the possibility that only a very few of the gastric tubules or parietal cells are 
functionally active at any one time. The amount of the superficial deposit, 
frequently found in cases in which direct evidence of the occurrence of 
the reaction in the underlying tissue was not forthcoming, appears also 
to indicate that the Prussian blue, when formed, is removed from the tubules 
with great rapidity. The rapid removal of the Prussian blue, together with 
the variation in the functioning time of the several gland tubules, or of 
certain cells within these, may be largely responsible for the difficulty 
encountered in obtaining evidence of a positive character within the 
glandular structures, and for the lack of success experienced by earlier 
investigators. 
Factors, the nature of which is unknown, are also present, which under 
certain conditions, equally unknown, influence the secretion of the acid 
in such a manner that it is no longer directed entirely towards the free 
surface, and consequently the Prussian blue may be found in situations 
other than those hitherto named. 
Proof of this was obtained in two rabbits (Rabbits 5 and 6, see Table). The 
remarkable quantity of Prussian blue found in the case of Rabbit 5 adhering 
to the surface of the gastric mucosa at the end of an experiment of 64 hours’ 
duration has already been referred to. (For experimental data see pp. 77—79.) 
In Rabbit 6, which was killed 3 hours after receiving the first, and 
between three-quarters of an hour.and one hour after the last injection, the 
