226 A. B. MACALLUM. 
In the rods and cones of the retina in Amblystoma and 
Necturus an iron reaction was frequently obtained like that 
represented in fig. 37. It was always feeble and confined to 
the trabeculz, which stretched across the long axis of the rod, 
or which formed the network in the cones. In some cases (as 
in fig. 87, a) pigment-granules were observed attached to the 
rods, probably derived from the cells of the tapetum nigrum, and 
as the pigment probably contains iron, it is uncertain whether 
the iron demonstrated in the rods and cones was not derived 
by diffusion of some iron-holding substance from this source. 
The eleidin granules in the stratum granulosum in the 
human skin give, after treatment of sections of the epidermis 
with sulphuric acid alcohol, a dark green reaction with ammo- 
nium sulphide. I have not succeeded in obtaining a reaction 
for iron in them when the containing cells, hardened in alcohol, 
were simply subjected to the prolonged application of the gly- 
cerine and sulphide mixture in the warm oven. Since the 
chromatin of the nuclei in the underlying stratum mucosum 
is, as elsewhere, iron-holding, while the nuclei in the stratum 
granulosum are poor in chromatin, it is not improbable that 
the iron, at least of that part of the latter which disappears 
from the nuclei, is the source of the iron shown in the eleidin 
granules. The homogeneous substance constituting the stratum 
lucidum also gives a reaction for iron, which is diffuse and less 
marked than in the granules of the underlying layer. 
In my observations on preparations of the human thyroid 
and of that of the dog, although it was easy to demonstrate the 
presence of iron in the nuclear chromatin, and to a certain 
extent in the cytoplasm of the cells lining the alveoli, I did 
not succeed in finding any of it in the “colloid” matter. 
Under certain conditions this substance absorbs staining 
matters, and it also gives! the molybdate-pyrogallo] reaction 
of Lilienfeld and Monti.” These facts suggest that the colloid 
1 F, Gourlay, “The Proteids of the Thyroid and the Spleen,” ‘Journal of 
Physiology,’ vol. xvi, p. 28, 1894. 
2 “Die mikro-chemische Lokalization des Phosphors in den Geweben,” 
‘Zeit. fiir Physiol. Chemie,’ vol. xvii, p. 410, 1893. 
