STRUCTUEE OF A SPORE OF MILDEW. 57 



pendent in great measure upon change in the external 

 conditions. When these are favonrable, the bioplasm 

 of the amcfiba and the mucus corpuscle grows very fast 

 and multiplies rapidly, but when the external condi- 

 tions are unfavourable, and the supply of pabulum 

 very Hmited, the bioplasm ceases to increase rapidly, 

 and becomes changed upon the surface ; a firm mate- 

 rial being produced which protects the living matter 

 within from destruction, but which renders its free 

 movement impossible. Under such circumstances, 

 the so-called mucus corpuscle may assume the cha- 

 racters of an epithehal cell. 



But the growth of bioplasm and the production of 

 the formed material can be so well studied in the 

 lower fungi, that I shall venture to draw attention to 

 the phenomena as they occui' in a specimen of this 

 lowly organism before alluding to the change as it 

 occurs in man and animals. 



90. Structure of a spore of mildew. — If one of the 

 simplest structures — the microscopic sporule, which is 

 so light that ib may be wafted long distances by 

 ctirrents of air — be examined, we shall find that it is 

 not the same in every part. It consists externally of 

 a delicate transparent, glass-like texture, and within 

 of a material having ia very faintly granular ap- 

 pearance. In order to demonstrate thi^s fact, a little 

 ordinary mildew dust, which is one of the lowest 

 forms of existence, may be examined. The little 

 round bodies which compose it are comparatively 

 large, and well suited for investigation. They may 

 be studied in glycerine under a twelfth of an inch 

 object glass. Each of these coiTesponds to a single 

 cell or elementary 2:>art of the more complex tissues. It 

 has a tolerably thick well-defined outline, while the in- 

 terior is perfectly transparent. When this transparent 

 matter is expressed and placed under very high magni- 

 fying powers, numerous very minute particles like dots 

 will be observed. Here then are two kinds of ma- 



