INTRODUCTION. 



THE micro-organisms causing putrefaction, fermentation, and in- 

 fectious diseases belong partly to the lower vegetable, and partly to 

 the lower animal kingdom, and are divided into four groups : 



1. Hyphomycetes^ or Mould Fungi. 



2. Blastomycetes, or Yeast Fungi. 



3. Schizomycetes, or Bacteria. 



4. The Protozoa. 



1. The Hyphomycetes consist of cells multiplying only by growth 

 of the distal or point cells, and in this manner forming threads or 

 hyphae, i.e., chains of cells. The fully-developed mould consists of a 

 mycelium (which might be compared to the roots of higher plants), 

 and of fruit hyphse which develop out of the former, and bear on 

 their extremities the spores or conidia. The moulds in the absence of 

 oxygen frequently form cloudy mycelial masses within the culture 

 media. Spores are only formed on the surface of media in contact 

 with oxygen. The Hyphomycetes possess no chlorophyll. 



2. The Blastomycetes consist of ovoid or round cells, multiplying by 

 bud-like processes from the mother cells, also at times by spore 

 formation. There is reason to believe they are allied to the mould 

 fungi. If the newly-formed cells are not detached, a conglomerate 

 cell of buds is formed. On unfavourable, strongly alkaline, and sugar- 

 free media a mycelium is developed. 



3. The Schizomycetes or Bacteria are a large group of unicellular 

 organisms developing from pre-existing cells of the same species ; they 

 never appear spontaneously, and multiply by a process of transverse 

 division. 



They are spherical, oval, rod-like, and spiral in shape, and are 

 generally devoid of chlorophyll or green colouring matter possessed 

 by the higher plants ; owing to this circumstance they are forced to 

 obtain their nutritive materials from organic matters, and therefore 

 exist in either saprophytic or parasitic conditions. 



A 



