45^ THIRD PART. — BACTERIA OR SCIIIZOMVCETES. 



a. Regard being had solely to the shape of the isolated cells or to 

 their simplest genetic union, 



i. Cocci : isolated cells which are isodiametric or at leasl very slightly elongated 

 in one direction. These are distinguished when necessary according to their 

 dimensions into micrococci, macrococci and monad-forms. 



2. Rod-like forms : cells elongated in one direction and cylindrical, rarely 

 fusiform, isolated or in a short chain. These again are distinguished into 

 short rods {Bacteria), long rods {Bacilli), fusiform rods {Clostridia) and some others. 



3. Spiral forms: spirally twisted rods, some with narrrow coils {Spirillum, 

 Spirochaete), some with distant and very steep coils {Vibriones). 



It follows necessarily from what has been already stated that no sharp line of 

 distinction as regards their shape can be drawn between short rods, for instance, and 

 cocci, or between a slightly twisted Vibrio and a Bacillus which departs to a trifling 

 extent only from a mathematically straight line ; nor can they at present be always 

 clearly distinguished by their structure. 



This is especially the case with respect to the rod-like forms, since a rod may be 

 a single cell of the corresponding shape, or a number of cells firmly connected together 

 and closely related to one another genetically. In the latter case while the cell is 

 dividing repeatedly, the partition-wall may be of so delicate a structure that the com- 

 pound body, if not carefully examined, may be taken for a simple homogeneous 

 body. Hence when these organisims are simply spoken of as rods we must under- 

 stand that the writer is alluding to their outward appearance only, unless the structure 

 also is exactly described. 



To these three kinds must be added a fourth, namely the swollen bladder-like 

 forms. Individuals of this kind are found in company with the other three and are 

 evidently produced from them ; they are distinguished by having their cell swollen to 

 several times the size of the other form, with a knobbed and irregular outline. 

 These inflated forms have been observed in artificial cultivations where the nutrient 

 sul stances are in insufficient quantity or are exhausted; Zopf and Cienkowski found 

 them in Cladothrix and Crenothrix, Buchner and Prazmowski in forms of Bacillus, 

 and Xcelsen in Bacterium cyanogenum. They are therefore considered to be 

 diseased states of the other forms, and have been termed by Nageli and Buchner 

 involution-forms. Hansen, on the other hand, has shown that they occur very 

 frequently, indeed almost invariably, with the Bacteria of mother of vinegar ; we do 

 not know whether they have not some further meaning in this and perhaps also in 

 most other cases. 



b. According to the mode of the connection between the individual 

 cells, each of the above form-groups may be, 



1. free, that is not firmly joined together, though occurring in the society of 

 great numbers of like individuals. 



2. in the form of filaments, that is joined together and forming long filiform 

 rows. These filaments are unbranched in most Schizomycetes and then the form is 

 known as Leptothrix or Mycothrix ; they are branched in a few cases only {Cladothrix). 

 In the latter form one extremity of a cell bends outward from the row in which 

 1: occurs, and continues its growth and divisions in a divergent direction. The 



