10 ANIMAL PHYSIOLOGY. 
The yeast-cell is now believed to possess a nucleus. 
Chemical.— When yeast is burned and the ashes analyzed, 
they are found to consist chiefly of salts of potassium, calcium, 
and magnesium. 
The elements of which yeast is composed are C, H, O, N, 8, 
P, K, Mg, and Ca; but chiefly the first four. 
Physiological.—If a little of the powder obtained by drying 
yeast at a temperature below blood-heat be added to a solution 
of sugar, and the latter be kept warm, bubbles of carbon di- 
oxide will be evolved, causing the mixture to become frothy; 
and the fluid will acquire an alcoholic character (fermenta- 
tion). 
If the mixture be raised to the boiling-point, the process de- 
scribed at once ceases. 
It may be further noticed that in the fermenting saccharine 
solution there is a gradual increase of turbidity. All of these 
changes go on perfectly well in the total absence of sunlight. 
Yeast-cells are found to grow and reproduce abundantly in 
an artificial food solution consisting of a dilute solution of cer- 
tain salts, together with sugar. 
Conclusions.— What are the conclusions which may be legiti- 
mately drawn from the above facts ? 
That the essential part of yeast consists of cells of about the 
size of mammalian blood-corpuscles, but with a limiting wall 
of a substance different from the inclosed contents, which latter 
is composed chiefly of that substance common to all living 
things—protoplasm ; that like other cells they reproduce their 
kind, and in this instance by two methods: gemmation giving 
rise to the bead-like aggregations alluded to above; and in- 
ternal division of the protoplasm (endogenous division). 
From the circumstances under which growth and reproduc- 
tion take place, it will be seen that the original protoplasm of 
the cells may increase its bulk or grow when supplied with 
suitable food, which is not, as will be learned later, the same in 
all respects as that on which green plants thrive; and that this 
may occur in darkness. But it is to be especially noted that the 
protoplasm resulting from the action of the living cells is 
wholly different from any of the substances used as food. This 
power to construct protoplasm from inanimate and unorgan- 
ized materials, reproduction, and fermentation are all proper- 
ties characteristic of living organisms alone. — 
It will be further observed that these changes all take place 
within narrow limits of temperature; or, to put the matter 
