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 fi'ustule 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 of the rim. 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 Diatomaceae. 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-Avall, 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 per cent. 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 Naviculse 

 these plates, two in number, lie 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 towaixls the median lino, lea^dng 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 



