CHAPTER II 



MORPHOLOGY AND RELATIONSHIPS OF MICROORGANISMS 

 CONCERNED IN DISEASE PRODUCTION 



POSITION OF PATHOGENIC MICROORGANISMS 



Differentiation of Animals and Plants. The distinctions we 

 commonly recognize as differentiating plants from animals in 

 large measure disappear among the microscopic forms of life. 

 It is worth while, therefore, to discuss the factors that are taken 

 into consideration in the assignment of a particular microorgan- 

 ism to the animal or the plant kingdom. The difficulty arises 

 particularly in the differentiation of the bacteria and the protozoa. 

 Bacteria, as will be shown later, probably have in all cases a cell- 

 wall which in function is closely related to that of plants. A 

 cell-wall is frequently absent in protozoa, the limiting mem- 

 brane usually consisting of the ectoplast (see below) alone. The 

 composition of the cell-wall is in some cases like that of many 

 animals (chitin), in others like plants (cellulose). In shape 

 and habit of growth and reproduction the bacteria resemble 

 very closely certain undoubted plants among the blue-green 

 algae and the mold fungi much more than they do animal forms. 

 On the other hand, there are some types which intergrade with the 

 protozoa, so that there is at present doubt as to their correct 

 systematic position. In the matter of food supply and food 

 utilization some bacteria resemble higher plants, others resemble 

 animals. The possession of organs of motion by bacteria has 

 no direct bearing on the subject, inasmuch as undoubted plants 

 of the group of algae and many of the protozoa have them. The 

 evidence, on the whole, is much in favor of classification of the 

 true bacteria with plants. 



Before beginning a study of the morphology or structure of 

 these microorganisms, their position and relationships to the 

 other groups of plants and animals should be understood. Fol- 



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