150 GENERAL BIOLOGY OF MICRO-ORGANISMS 



or fatty substance and surrounded by a membrane. Very early in 

 the process the spore no longer stains readily. In some forms 

 (Bact. anthracis) the cell in which a spore has formed disintegrates 

 rapidly, setting free the spore, while in others (B. telani) the cell 

 may continue its activities after formation of the spore. The spore 

 germinates when conditions again become favorable to active 

 growth. The new cell may burst the spore wall into halves, or 

 at the end, or the spore wall may soften and become a part of the 

 new growing cell. 



Filterable Viruses. The difficulty of accurate morphological 

 study is so great as to appear insurmountable in the case of cer- 

 tain microbes which are very definitely recognizable by certain 

 effects which they produce. This is especially true of those 

 living things capable of passing through the fine filters which 

 prevent the passage of small bacteria. The causes of certain 

 diseases exhibit this character, and these have come to be known 

 as filterable viruses. There can be little question that non-patho- 

 genic filterable microbes also exist although they seem to have 

 escaped observation. Accurate knowledge of the morphology 

 of these forms remains to be disclosed by future investigation. 

 Meanwhile, the efforts to classify them as bacteria or as protozoa 

 may well be spared. The propriety of including them as living 

 things is, however, only occasionally questioned. 



Protozoa. The protozoa or unicellular animals have assumed 

 very great importance as causes of disease during the past dec- 

 ade. Fortunately for the systematist, the free-living protozoa 

 had received considerable careful study and the larger groups of 

 protozoa had been well defined before the interest in pathogenic 

 properties had the opportunity to over-shadow morphological 

 study. The number and variety of easily recognizable morpho- 

 logical characters presented by the protozoa are greater than 

 those of the bacteria; and the organisms are, on the whole, 

 larger. These factors make for more accurate observations of 

 morphological characters, and their more successful employment 

 as a basis of classification. 



