238 Proceedings of the Royal Irish Academy. 
priately assimilated to a pill-box, consisting of the box itself and the 
cover which slips over it; and if we suppose the cover to be of the 
same depth as the box, or nearly so, we have a structure on a 
large scale which nearly resembles the silicious covering of such 
Diatomaceous species as possess a circular form, and which with some 
modification may be taken to illustrate the general plan on which the 
silicious part of the cell of Diatoms is constructed. In the larger 
forms it may easily be noticed that one valve of the frustule, with its 
accompanying rim or hoop, is smaller than the other into which it fits, 
as the slides of a telescope fit into one another. It has been supposed 
that in the parent frustule the two valves are of the same size, and that 
the diminution in the dimensions of one valve is owing to the fact of its 
being developed within the rim of the primary valve, and is conse- 
quently smaller than it by the thickness oftherim. Pfitzer, however, 
has remarked that in some cases at least the difference in size is 
noticeable in the mother cell, in which one valve is secreted in the 
first instance, and then the opposite valve is formed within the former. 
This remark is worthy of notice and should be borne in mind when 
cases of conjugation come under view, in order to ascertain whether 
the occurrence is casual, or whether the same process takes place in 
the other species of Diatomacez. To the distinguished author just 
named belongs the merit of having contributed more than any other 
to the extent and accuracy of our knowledge concerning the various 
parts and disposition of the cell-contents. There is first the plasm-sac, 
consisting of a fine colourless plasm, forming a closed sac of the shape 
of the cell, and in which the cell-contents are enveloped. It is often 
very difficult for the observer to.make himself certain of the existence 
of this sac, because its refractive power differs but slightly from that 
of water, but the structure becomes apparent immediately on the 
application of dilute hydrochloric acid. The effect of this re-agent 
is to produce an instantaneous contraction of the sac, which at first, as 
it recedes from the cell-wall, preserves the form of the cell and still 
maintains connexion with it by means of a few pellucid threads, but 
after some time it becomes contracted into a round mass. This result 
is accomplished most effectively by the use of osmic acid at the 
strength one percent. Iodine gives a bright yellow colour to the 
plasm-sac. Within the plasm-sac, and in close proximity to it, is the 
structure to which Pfitzer has given the name of Endochrome- 
plates, varying in number and position in the various genera. 
Some possess two of these plates, others only one. In the Nayicule 
these plates, two in number, le one at either side, the middle of the 
plate corresponding with the middle of the hoop or connecting band, 
whence they pass on either side towards the median line, leaving a 
small narrow space down the middle of the valve free; in other genera 
there is but one such plate, variously disposed. They consist of a 
thick substance, and are of the same colour thoroughout, varying from 
light yellow to dark yellowish brown. The plasm of which these 
plates consist differs in density from the plasm whioh forms the 
