172 GENERAL BIOLOGY OF MICRO-ORGANISMS 



daughter cells. The presence of unfavorable influences, such as 

 antiseptics or bacterial waste products in the medium, may cause 

 marked irregularities in the shape and size of the cells, so-called 

 involution forms. The ability to form endospores may be lost 

 through growth at high temperature. The form which a micro- 

 organisms presents in a given instance may not, therefore, be 

 regarded as essentially typical without regard to the conditions 

 under which it has been produced. 



The morphology of cell-groups is even more obviously depend- 

 ent upon the conditions of the environment and the physiological 

 properties of the micro-organism. A slow scanty growth on a 

 given medium does not necessarily mean that the organism 

 essentially lacks vigor. It may mean that the medium is not well 

 adapted to the requirements. Diffuse growth through a semi- 

 solid medium may be merely an expression of the motility of an 

 organism. A great variety of different culture media have been 

 employed to bring out more or less characteristic features in the 

 gross appearance of cultures, and these appearances often depend 

 upon the grouping of the cells or upon their fermentative activity 

 or both. Although the characters of a cell-group of micro-organ- 

 isms are really morphological characters of the same general na- 

 ture as the morphological characters of higher plants and animals, 

 to which so much significance is attached, in the case of micro- 

 organisms in an artificial environment, such as a culture medium, 

 the gross appearance or the cell-grouping is more properly regarded 

 as a feature of physiological rather than morphological significance. 

 Nutrient gelatin is a medium well adapted, in the case of those mi- 

 crobes which will grow in it, for showing physiological differences 

 in the appearance of cell-groups or colonies, and perhaps a greater 

 variety of appearances may be obtained upon this medium than 

 any other. Unfortunately its use entails certain difficulties, the 

 most important of which is the necessity for experience and s care 

 in the interpretation of the appearances observed. Important 

 features in the appearance of the colonies and other cell-groups 

 are brought out by the use of various other media. 



