1 94 GROWTH AND VITAL CHANGES 



of a material having a faintly granular appearance. Suppose 

 a little ordinary mildew dust, which is one of the lowest 

 forms of existence possessing a very simple structure, be 

 examined. The little round bodies which compose it are 

 larger than those above referred to, and will therefore suit 

 the purpose of investigation better. Each of these has a 

 tolerably thick well-defined outline, while the interior is 

 perfectly transparent. When this transparent matter is ex- 

 amined under very high magnifying powers, numerous very 

 minute particles like dots will be observed. We have then 

 to notice that the spore of mildew consists of two parts, 

 i, a protecting capsule, and 2, included matter. The first, 

 situated externally, is firm, and glass-like, and arranged so 

 as to form an investing membrane closed at all points. The 

 other lying within, is softer, probably semifluid, hardly 

 visible from its extreme clearness and translucence, and 

 exhibits no form or structure whatever. 



Now, if such bodies be placed under favourable con- 

 ditions, certain changes will occur. Let some spores be 

 put, for instance, upon the moist surface of a glass slide, 

 and after a time let the slide be placed under the micro- 

 scope. The particles will soon absorb moisture, and swell 

 up, and the membrane will be seen to have become thinner 

 in proportion to the whole mass, while the matter within has 

 increased in amount Next, a change may be observed, in 

 some cases, to have taken place at one point in the mem- 

 brane. A small orifice is seen, through which a little of 

 the granular contents of the body, covered with a thin layer 

 of the inner part of the membrane, slowly makes its way, 

 and thus a small nodule is formed from, and is continuous 

 with, the matter within, and tends to pass through the 



