the I 7 east Plant. 
49 
Our recent observations show that there is a closer connexion between 
the nuclear vacuole and the nucleolus than was formerly supposed, and we 
have come to the conclusion, as the result of a careful study of their 
structure and position in the cell and their reactions towards nuclear 
reagents and stains, that they definitely constitute the nucleus of the 
Yeast Plant. 
Examination of Living Cells. 
The microscopic examination of healthy yeast cells in a resting condi¬ 
tion shows that they contain a central well-marked vacuole with hyaline 
contents, and usually one large or two or three smaller refractive granules, 
which exhibit the characteristic Brownian movement (PI. VI, Figs. 1 - 3 ). 
The vacuole varies very much in size, in some cases filling the cell almost 
completely, in others being much smaller and sometimes hardly visible. The 
cytoplasm forms a more or less hyaline layer round the vacuole, and there are 
present in it a varying number of highly refractive granules, which are 
commonly arranged round the outside of the vacuole or vacuoles, if more 
than one is present, but are sometimes irregularly disposed in groups 
(PI. VI). The uniform presence of the vacuole, its distinct appearance, and 
the constant presence of bright, refractive granules in it, indicate that it 
plays some important part in the cell economy. The nucleolar body 
(nucleolus) is invisible in perfectly healthy cells, though some indication 
of its presence is occasionally given by the position of the refractive 
granules (Fig. 2 ). If the cells lose some of their turgidity through the 
pressure of the cover-glass, the position of the nucleolus is often indicated 
by a depression in the vacuole; occasionally under these conditions it can 
be distinguished as a slightly refringent spherical body in contact with the 
central vacuole. 
On the addition of a dilute aqueous solution of methylene blue or 
gentian violet, the nucleolus immediately shows as a lightly stained homo¬ 
geneous body lying between the vacuole and the cell-wall, and in close 
contact with the former (Pig. 5 ). A number of the refractive granules in 
the cytoplasm, together with the moving granules in the vacuole, stain 
deeply with both methylene blue and gentian violet, and are therefore 
probably volutin granules (Figs. 5 - 7 ). Others stain black with osmic acid, 
and are doubtless globules of oil; an occasional granule may give the 
glycogen reaction with iodine, but there still remains a number of these 
highly refractive granules which are not accounted for. 
If a culture of such a yeast is started in Pasteur’s solution, it is found 
that after a few minutes the vacuole contracts (Figs. 8, 9). In the course 
of 20-30 minutes it regains its turgid appearance and is particularly distinct, 
although it has become somewhat smaller (Fig. 10). The vacuole then 
becomes less visible (Figs. 11, 12), till at the end of an hour it has apparently 
E 
