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and unstable. The nucleus plays a prominent part in the life-history of the cell, and seems to be essential to its perfect development and greatest physiological efficiency.

UNICELLULAR PLANTS.

Yeast (Tonda, Saccharomyces Cerevisiœ).

Fig. 2.—Various stages in the development of brewer's yiest, seen, with the exception of the first in the series, with an ordinary high power (Zeiss, D. 4) of the microscope. The first is greatly magnified (Gundlach's 1/16 immersion lens). The second series of four represents stages in the division of a single cell; and the third series a branching colony. Everywhere the light areas indicate vacuoles.

The essential part of the common substance, yeast, may be studied to advantage, as it affords a simple type of a vast group of organisms of profound interest to the student of physiology and medicine. To state, first, the main facts as ascertained by observation and experiment:

Fig. 3.—The endogonidia (ascospore) phase of reproduction—i.e., endogenous division.

Morphological.—The particles of which yeast is composed are cells of a circular or oval form, of an average diameter of about 1/3000 of an inch.

Fig. 4.—Further development of the forms represented in Fig. 3.

Each individual torula cell consists of a transparent homogeneous covering (cellulose) and granular semifluid contents (protoplasm). Within the latter there may be a space (vacuole) filled with more fluid contents.

The various cells produced by budding may remain united like strings of beads. Collections of masses composed of four or more subdivisions (ascospores), which finally separate by rupture of the original cell wall, having thus become themselves independent cells, may be seen more rarely (endogenous division).