18 COMPARATIVE PHYSIOLOGY. 



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 examr 

 pie, 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. 



The Bacteria. 



The bacteria include numberless varieties of organisms of 

 extreme minuteness, many of them visible only by the help of 

 the most powerful lenses. Their size has been estimated at 

 from ^TOT to ioLo of ^^ inch in diameter. 



They grow mostly in the longitudinal direction, and repro- 

 duce by transverse division, forming spores from which new 

 generations arise. 



Some of them have vibratile cilia, while the cause of the 

 movements of others is quite unknown. 



As in many other lowly forms of life, there is a quiescent 

 as well as an active stage. In this stage (zooglaea form) they 

 are surrounded by a gelatinous matter, probably secreted by 

 themselves. 



Bacteria grow and reproduce in Pasteur's solution, rendering 

 it opaque, as well as in almost all fluids that abound in proteid 



