Glycogen is formed by the action of a ferment on the carbohydrates - the starches being converted into sugars.
Within the cytoplasm are found manifestations of functional activity, in the form of digestive vacuoles, granules, fat, glycogen, pigment, and foreign bodies.
The significance of glycogen in large amounts, or of its absence from the tissues in pathological conditions, is not clearly understood.
Invisible to the microscope, but rendered visible by reagents, are glycogen, Mucor, Ascomycetes, yeast, &c. In addition to these cell-contents we have good indirect evidence of the existence of large series of other bodies, such as proteids, carbohydrates, organic acids, alkaloids, enzymes, &c. These must not be confounded with the numerous substances obtained by chemical analysis of masses of the fungus, as there is often no proof of the manner of occurrence of such bodies, though we may conclude with a good show of probability that some of them also exist preformed in the living cell.
In other forms a substance (probably glycogen or amylo-dextrin) which turns brown with iodine has been observed.
Arsenic and antimony do not form combinations with albumen, but they both greatly depress the central nervous system and circulation; and, if their action be long continued in large doses, they cause fatty degeneration of the viscera and disappearance of glycogen from the liver.
Glycogen, a substance related to starch and sugar, is found in the Fungi and Cyanophyceae as a food reserve.