IRON COMPOUNDS IN ANIMAL AND VEGETABLE CELLS. 2383 
forming the two adjacent bands. The fibrils, or what are in 
appearance such structures, have so little iron in them that 
frequently in a large part of an isolated filament their blue 
reaction may not be sufficiently deep to betray their presence, 
but the chances of observing them may be increased by staining 
such preparations carefully with safranin. Probably the ex- 
pression fibril is not a correct one to apply to these appear- 
ances, for they may be the optical sections of the partition 
walls of compartments, the extreme ends of which would in 
that case be formed by the dim bands. What appears to sup- 
port the latter view is the fact that in some of the thickest dim 
bands the Prussian blue reaction reveals the presence of a single 
row of vesicles extending from one end of the band to the 
other, the vesicles sometimes having an elongated form parallel 
with the filament. It seemed to me that these were the 
initial stages in the division of one dim band into two, that 
the thinner bands represent those most recently formed, and 
that, therefore, the vesicular mode of formation would result in 
the production of a series of compartments the thin walls of 
which, in the clear bands, would appear as fibrils. The struc- 
tures observed are, however, so exceedingly minute that it is 
impossible to determine definitely anything on this point. 
The iron-holding substance in the filament is, therefore, dis- 
posed in the rodlets of the dim band and in the fibril-like 
elements connecting the rodlets of one dim dise with those of 
its neighbours. The only exception to this statement may be 
made in regard to the structure of the swellings which are 
sometimes found on the course of a filament (fig.50). In this 
case the dim discs are replaced by an iron-holding reticulum 
disposed in the interior of the swollen portion of the filament. 
A comparison of this portion with the adjacent portions 
of the filament appears to indicate how the reticulum has 
arisen and what its relations are. The iron-holding bands 
on either side are less regular in their disposition than else- 
where, and the fibril-like structures arising from them appear 
to be directly connected with the iron-holding substance of the 
reticulum referred to. The swollen portion of the filament 
