ings and spiral thickenings and pits, offered a
fascinating field of enquiry.

The idea that the cell-contents might show a characteristic and
individual structure had hardly dawned upon botanists when Schleiden
published his famous paper, _Beitraege zur Phytogenesis_.[240] Schleiden's
theme in this paper is the origin and development of the plant cell, a
subject then very obscure, in spite of pioneer work by Mirbel. A few
years before, Robert Brown had called attention to the presence in the
epidermal cells of orchids and other plants of a characteristic spot
which he called the areola or nucleus.[241] Schleiden saw the importance
of this discovery, confirmed the constant presence of the nucleus in
young cells, and held it to be an elementary organ of the cell. He named
it the cytoblast because, in his opinion, it formed the cell. It was
embedded in a peculiar gummy substance, the cytoblastem, which formed a
lining to the cellulose cell-wall. Within the nucleus there was often a
small dark spot or sphere--the nucleolus. The nucleus, Schleiden
thought, originated as a minute granule in the cytoblastem which
gradually increased in size, becoming first a nucleolus (_Kernchen_),
and then, by further condensation of matter round it, a nucleus. Several
nuclei might be formed in this way in a single cell. New cells took
their origin directly from a full-grown nucleus, in a peculiar way which
Schleiden describes as follows:--"As soon as the cytoblasts have reached
their full size a delicate transparent vesicle arises on their surface;
this is the young cell, which at first takes the shape of a very flat
segment of a sphere, of which the plane surface is formed by the
cytoblast, the convex side by the young cell itself, which lies upon the
cytoblast like a watch-glass on a watch" (p. 145). The young cells
increase in size and fill up the cavity of the old cell, which is in
time resorbed. Cell-development always takes place within existing
cells, and either one or many new cells may be formed within the
mother-cell. Schleiden's views on cell-formation were drawn from some
rather imperfect observations on the embryo-sac and pollen-tube, but he
extended his theory to cell-formation in general. Though wrong in almost
all respects the theory had at least the merit of fixing attention upon
the really important constituents of the cell, the nucleus and the
cell-plasma. To Schleiden, too, we owe the conception of the cell as a
more or