34 MORPHOLOOY 



scopic cultures of organisms of the coli-typhoid group, and have observed multiplication 

 by elongation and fission from all three points of the Y. (See also Wyckoff 1934.) Whether 

 the occurrence of these forms has any special significance is a question that the future must 

 determine. 



Spore-formation, which has been described above, is confined to certain species 

 of bacteria. The spore constitutes a special resting and resistant phase of the 

 bacterial cell ; its formation appears to be determined in large part by environ- 

 mental factors, and there is little ground for regarding it as an integral part of any 

 periodic or essential life-cycle (but see p. 26). 



A question still at issue is the occurrence of other forms of reproduction, and 

 in particular of the existence or non-existence of a complex life-cycle, in which 

 sexual processes may or may not play a part. It is not possible to discuss this 

 problem at all fully. It must, however, be noted that there are those who believe 

 that such a life-cycle is a natural characteristic of all bacterial species, and that 

 multiplication by binary fission, which appears to be the only mode of reproduction 

 in young, actively growing cultures, is only one phase of the cycle, which we encour- 

 age by the particular artificial conditions under which we choose to study bacterial 

 cells. 



Jones (1913, 1914, 1920) described reproduction by gonidia-hke bodies in the case of 

 Azoiobader. These observations were greatly extended by Lohnis and Smith (1916) who 

 described a far more complex life-cycle in this, and in other, species, and appUed their 

 results to the reproduction of bacteria in general (Lohnis 1921). Stoughton (1929, 1932) 

 in describing the morphology and cjrtology of Bad. mahaceanim, an organism producing 

 angular leaf-spot in the cotton plant, records the formation, by budding from the bacillary 

 form, of coccus-like bodies that subsequently develop into bacilli, and also the fusion of 

 bacillary cells with the formation of zygospores. 



Almquist described various forms of reproductive bodies, mauily minute gonidial 

 structures, in a wide range of bacteria (Almquist 1893-1924, Almquist and Koraen 1918, 

 Koraen 1918). It may be noted that there was a general tendency for these appearances 

 to be observed m old cultures, and sometimes in cultures which had been allowed to undergo 

 a marked degree of drying. However, Allen, Appleby and Wolf (1939) observed the 

 relatively early liberation of intracellular granules from growing vegetative cells of their 

 bacUlus. A little later, minute rods were found in the culture, which appeared to have 

 originated in the granule, and which later developed into the modal form of the bacUlus. 

 Mellon (1917-26) has described a variety of forms which he regards as stages in the com- 

 plex life-cycle of various bacterial species. Enderlem (1925) upholds a similar thesis. 

 Kuhn (1929, 1930) has described a remarkable series of morphological forms in common 

 organisms, some of which are interpreted as evidence of a life-cycle, others of which — • 

 the so-called Pettenkofer bodies — are regarded as intracellular parasites. Hadley (1927 ; 

 see also HadW, Delves and Klimek 1931) in a detailed review, which should be consulted 

 by those who desire further information on this subject, sets out the available data with 

 regard to the occurrence and possible significance of these various forms, and seeks to 

 relate them to the phenomenon of bacterial dissociation, which we shall discuss in a sub- 

 sequent chapter. A shorter discussion of the possible modes of bacterial reproduction 

 will be found in papers by Bergstrand (1920, 1921, 1923). 



It is, we think, impossible on the evidence at present available to come to any 

 conclusion on the general problem at issue. That a cycle of morphological changes 

 may characterize the life-history of a particular bacterial species under natural con- , 

 ditions there can be no doubt. The detailed studies of Bewley and Hutchinson 

 (1920) and of Thornton and Gangulee (1926) on Rhizobiuni leguminosarum provide 

 a case in point, though their conclusions have been questioned by Lewis (1938). 



