Page:A Text-book of Animal Physiology.djvu/39

 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.

(Tonda, Saccharomyces Cerevisiœ).



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:



Morphological.—The particles of which yeast is composed are cells of a circular or oval form, of an average diameter of about $1⁄16$ of an inch.



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).