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the other hand, in a mainly physiological system morphologically 

 related groups will be widely dispersed. 



Guided by these general considerations we shall first of all give a 

 survey of the morphological units which can be distinguished in the 

 bacterial kingdom and of the way in which they seem to be mutually 

 related. This will be followed by the differentiation of the morphol- 

 ogically homogeneous groups on the basis of katabolism. 



It seems acceptable that the diversity of bacterial forms is the out- 

 come of various independent morphological evolutions which have 

 had their starting-point in the simplest form both existent and con- 

 ceivable: the sphere. 



The existing forms suggest that four such evolutionary lines have to 

 be distinguished. 



a. In the first place we can observe that certain bacteria in which 

 the spherical form is still fully maintained display a tendency to form 

 complexes of a more or less regular appearance. Whereas little signif- 

 icance can be attached to the diplococcus form since its occurrence is 

 inevitable in the reproduction process of the coccus, the formation of 

 chains of four or more cocci is typical for several species. Obviously in 

 this case one direction is preferential in cell division, the causative polar- 

 ity becoming manifest owing to the fact that the cells remain attached. 



In those spherical bacteria in which division takes place in two 

 directions which meet at right angles complexes may originate which 

 possess the typical form of tetrads or tetracocci. If finally the division 

 occurs in three directions of space the outcome may be the formation 

 of regular packets or sarcinae. 



It must be emphasized, however, that the mentioned morphological 

 groups comprise several transitional forms so that their delimitation, 

 although desirable, is subject to many difficulties. 



The occurrence of both Gram-positive and Gram-negative represent- 

 atives among the coccoid forms indicates a micromorphological evolu- 

 tion which, although its direction cannot be traced, justifies a differen- 

 tiation according to this character. 



The highest developmental stage in the group of spherical organ- 

 isms is in all probability displaid by the cocci able to form endospores, 

 the existence of which has recently been firmly established by an in- 

 vestigation of Gibson [1935]. 



b. Motility occurs only sporadically among the cocci, yet it in- 



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