The cell-walls are _impregnated with silica_ to such a degree that they are imperishable and are therefore able to contribute in a great measure to the formation of the earth’s crust. The structure of their cell-wall is most peculiar and _differs from all other plants_ (except certain Desmidiaceæ); it does not consist of a single piece but is made up of two--the “shells”--(compare _Exuviella_ and _Prorocentrum_ among the Dinoflagellata) which are fitted into each other, one being a little larger than the other and embracing its edge, like a box with its lid (Fig. 14 _B_). The two parts which correspond to the bottom and lid of the box are known as _valves_. Along the central line of the valves a longitudinal _rib_ may often be found, interrupted at its centre by a small cleft (perhaps homologous with the cilia-cleft of the Dinoflagellata), through which the protoplasm is enabled to communicate with the exterior (Fig. 14 _A_). It is principally by reason of the valves, which bear numerous fine, transverse ribs, striæ or warts, etc. (Figs. 14, 15, 17), that the Diatomeæ have become so well known and employed as test objects in microscopical science. When the division takes place, the two shells are separated a little from each other, and after the cell-contents have divided into two masses, two new shells are formed, one fitting into the larger valve, the other one into the smaller valve of the original frustule. The latter cell (frustule) is thus, upon the whole, smaller than the mother-cell, and as the cells do not increase in size, some frustules are smaller than the ones from which they are derived, and thus, by repeated divisions, it follows that smaller and smaller frustules are produced. This continued diminution in size is, however, compensated for by the formation, when the cells have been reduced to a certain minimum, of _auxospores_, 2–3 times larger. These may either be formed _asexually_ by the protoplasm of a cell increasing, rounding off and surrounding itself with a new wall (_e.g. Melosira_) or after _conjugation_, which may take place with various modifications: 1. Two individuals unite after the secretion of a quantity of mucilage, and the valves then commence to separate from each other, on the side which the two individuals turn towards each other. The protoplasmic bodies now release themselves from their cell-wall, and each rounds off to form an ellipsoidal mass; these two protoplasmic masses (gametes) coalesce to form a zygote, the cell-nuclei and chromatophores also fusing together. The zygote increases in size, and surrounds itself with a firm, smooth, siliceous wall--the _perizonium_. The auxospores, whichever way they arise, are not resting stages. The germination of the zygote commences by the protoplasm withdrawing itself slightly from the cell-wall and constructing first the larger valve, and later on the smaller one; finally the membrane of the zygote bursts (_e.g. Himantidium_). 2.