d transparency, show beautifully their structure. Being
perfectly transparent, it is only necessary to mount them in water
and cover.

The young ovule (_I_, _J_) consists of a central, elongated body
(nucellus), having a single layer of cells enclosing a large central
cell (the macrospore or embryo sac) (_sp._). The base of the
nucellus is surrounded by two circular ridges (i, ii) of which the
inner is at first higher than the outer one, but later (_K_, _L_),
the latter grows up above it and completely conceals it as well as
the nucellus. One side of the ovule grows much faster than the
other, so that it is completely bent upon itself, and the opening
between the integuments is brought close to the base of the ovule
(Fig. 95, _L_). This opening is called the "micropyle," and allows
the pollen tube to enter.

The full-grown embryo sac shows the same structure as that already
described in _Monotropa_ (page 276), but as the walls of the
full-grown ovule are thicker here, its structure is rather difficult
to make out. The ripe stigma is covered with little papillae
(Fig. 95, _F_) that hold the pollen spores which may be found here
sending out the pollen tube. By carefully opening the ovary and
slightly crushing it in a drop of water, the pollen tube may
sometimes be seen growing along the stalk of the ovule until it
reaches and enters the micropyle.

To study the embryo a series of young fruits should be selected, and
the ovules carefully dissected out and mounted in water, to which a
little caustic potash has been added. The ovule will be thus
rendered transparent, and by pressing gently on the cover glass with
a needle so as to flatten the ovule slightly, there is usually no
trouble in seeing the embryo lying in the upper part of the embryo
sac, and by pressing more firmly it can often be forced out upon the
slide. The potash should now be removed as completely as possible
with blotting paper, and pure water run under the cover glass.

The fertilized egg cell first secretes a membrane, and then divides
into a row of cells (_N_) of which the one nearest the micropyle is
often much enlarged. The cell at the other end next enlarges and
becomes divided by walls at right angles to each other into eight
cells. This globular mass of cells, together with the cell next to
it, is the embryo plant, the row of cells to which it is attached
taking no furt