Page:A Text-book of Animal Physiology.djvu/47

 becoming thinned to the point of rupture. The development of these spores takes place in substantially the same manner as those of Penicillium. Sporangia developing spores in this fashion by division of the protoplasm are termed asci, and the spores ascospores. So long as nourishment is abundant and the medium of growth fluid, this asexual method of reproduction is the only one; but, under other circumstances, a mode of increase, known as conjugation, arises. Two adjacent hyphæ enlarge at the extremities into somewhat globular heads, bend over toward each other, and, meeting, their opposed faces become thinned, and the contents intermingle. The result of this union (zygospore) undergoes now certain further changes, the cellulose coat being separated into two—an outer, darker in color (exosporium), and an inner colorless one (endosporium).

Under favoring circumstances these coats burst, and a branch sprouts forth from which a vertical tube arises that terminates in a sporangium, in which spores arise, as before described. It will be apparent that we have in Mucor the exemplification of what is known in biology as "alternation of generations"—that is, there is an intermediate generation between the original form and that in which the original is again reached.

Physiologically the molds closely resemble yeast, some of them, as Mucor, being capable of exciting a fermentation.

The fungi are of special interest to the medical student, because many forms of cutaneous disease are directly associated with their growth in the epithelium of the skin, as, for example, common ringworm; and their great vitality, and the facility with which their spores are widely dispersed, explain the highly contagious nature of such diseases. The media on which they flourish (feed) indicates their great physiological differences in this particular from the green plants proper. They are closely related in not a few respects to an important class of vegetable organisms, known as bacteria, to be considered forthwith.