REPRODUCTION 33 



The nature of flagella, and the modes of generating and transmitting to them the 

 energy required for their propulsive effect, is largely unknown. Under fixed cultural 

 conditions, the speed of a flagellated bacterium is considerable, and to a certain extent 

 characteristic of the strain observed. The highest speeds are recorded for the cholera 

 vibrio, a monotrichate organism, which may travel more than 80 i-i per second (see Sanarelli 

 1919, Ogiuti 1936). 



Though flagella are usually confined to vibrios and bacilli, several observers have 

 described motile cocci (Schieblich 1932, KoblmiiUer 1935, Pownall 1935, Levenson 1938). 

 Under the electron microscope, flagella-like structures have been seen in spirochietes of 

 the genus Treponema (Morton and Anderson 1942). 



The results that have been obtained in the detailed study of antigenic structure 

 (see Chapter 8) have shown that the flagellar substance, or at least that portion 

 of it which forms the flagellar surface, is chemically distinct from the substances 

 forming the cell body. 



Involution Forms. — In old cultures of many bacteria forms may be found 

 which are quite unlike those seen in young and actively growing cultures. Some 

 of these forms are very typical of the bacterial species in which they occur, as is 

 the case with the so-called involution forms of the plague bacillus, and of some 

 other species of Pasteurella, and the giant forms which occur in cultures of the 

 meningococcus. The existence of these and other abnormal forms has long been 

 recognized, and they have been regarded as due to involution or degeneration 

 following the ageing of the culture, and the consequent unfavourable environmental 

 conditions. This view as to their nature has been generally adopted because of 

 the undoubted fact that, under the conditions in which they usually occur, the 

 majority of the organisms are non-viable ; while the extensive study of young 

 actively growing cultures has, in general, shown no more striking departure from 

 the normal form than the occurrence of unusually large cells, or occasionally in 

 the rod forms, of very long, almost filamentous cells, due apparently to delay in 

 cell division. 



Reproduction. — Although a description of the method of reproduction in 

 bacteria may be regarded as belonging more correctly to a discussion of their 

 physiology than of their morphology, yet the- question is so closely bound up 

 with dift'erences in form of the cells, and of the cell-aggregates which result from 

 successive division, that there are many advantages in considering the question 

 in the present chapter. 



There is no doubt at all that the method of multiplication of bacteria, under 

 optimal conditions of cultivation in the laboratory, is by simple binary fission, 

 each cell dividing into two daughter cells by constriction, with or without a well- 

 marked preliminary septation. 



In the spheroidal forms division may occur in any diameter. In the ovoid or 

 cylindrical forms division always occurs in the transverse diameter. It is never 

 longitudinal. 



In certain genera of bacteria, such as the Actinomyces, the formation of branching 

 filaments is a normal phase of growth and multiplication. In a few baciUary species, 

 and notably in C.diphthencB and 3Iyco. tuberculosis, rudivaentury branching has frequently 

 been described, and its occurrence in actively growing cells observed under the microscope 

 (HiU 1902). In most baciUary species such rudimentary branching is rare, and is seldom 

 observed in the examination of ordmary stained preparations. Hort (1917a, b), and 

 Gardner (1925) have, however, noted the occurrence of Y forms in actively growing micro- 



P.B. C 



