PARASITIC ORGANISMS. 17 
Fias. 17 to 28.—In the following Rgures, ha, denotes aérial hyphe ; sp, rangium ; zy, zy- 
gospore ; ex, exosporium ; my, mycelium ; mc, mucilage ; cl, columella ; en, endogonidia. 
Fia. 17.—Spore-bearing hyph of Mucor, growing from horse-dung. 
Fic. 18.—The same, teased out with needles (A, 4). 
Fias. 19, 20, 21.—Successive stages in the development of the sporangium. 
Fig. 22.—Isolated spores of Mucor. 
Fic. 23.—Germinating spores of the same mold. 
Fic. 24.—Successive stages in the germination of a single spore. 
Fies. 25, 26, 27.—Successive phases in the conjugative process of Mucor. 
Fie. 28.—Successive stages observed during ten hours in the growth of a conidiophore of Peni- 
cillium in an object-glass culture (D, 4). 
case becoming thinned to the point of rupture. The devel- 
opment 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 hyphe enlarge at the ex- 
tremities 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 de- 
scribed. It will be apparent that we have in Mucor the exem- 
plification of what is known in biology as “alternation of gen- 
erations”—that is, there is an intermediate generation be- 
tween 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, be- 
cause many forms of cutaneous disease are directly associated 
with their growth in the epithelium of the skin, as, for exam- 
ple, common ringworm; and their great vitality, and the facil- 
ity 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 differ- 
ences 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 forth- 
with. 
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