MORPHOLOGY 



789 



same culture, a heterogeneous mixture of cocco-bacilli, rods, and long threads. 

 Finally, though many strains retain their morphological characters unchanged for 

 weeks, months, or longer periods, others show quite definite changes in form after 

 a few cultures. WoUstein (1915) has noted the frequency with which the cocco- 

 bacillary type acquires the power of forming threads in artificial culture ; and there 

 has been a very general tendency to discard the old conception of a para-influenza 

 bacillus, differentiated on purely morphological grounds (Pfeiffer 1893), in favour of 

 the tentative view that such differences, unless associated with other well-marked 

 characters, must be disregarded by the systematist (Kristensen 1922). Smith 

 (1931) records a careful study of a series of strains isolated from the human naso- 

 pharynx. In almost every instance morphological variation occurred after a vary- 

 ing period of cultivation in artificial media, the most usual change being from 

 the short cocco-bacillary form to longer bacilli, or to curved or jointed filaments. 

 This change was in general associated with a change in colony form (see below). 



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Fig. 165. — H. influenzce. 



From 24-hours' culture on Fildes' agar, 

 atypical form, showing long, curved 

 bacillary forms ( X 1000). 



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Fig. 166. — H. influenzce. 



From 24-hours' culture on Fildes' agar, 

 showing thread forms, and large spherical 

 bodies ( X 1000). 



There is general agreement that it is possible to make a rough classification of 

 strains into two groups, " typical " and " atyj)ical " on the basis of morphology 

 alone ; and there are indications that, apart from certain strains isolated from cases 

 of meningitis (see Cohen 1909, Ritchie 1910, Henry 1912, Wollstein 1915), the 

 typical morphology predominates among strains isolated from pathological con- 

 ditions. The studies of Pittman (1931), which will be discussed more fully in 

 relation to the problem of antigenic structure, have emphasized the possible 

 importance of this distinction. But the evidence at present available suggests 

 that typical and atypical morphology are associated with a change presenting many 

 analogies to the smooth — >■ rough type of variation, rather than permanent char- 

 acters serving to differentiate stable varieties or types. 



The organisms of this genus form no spores and are, with the exception of 

 H. bronchisepticus, non-flagellated. They are usually described as non-capsulated, 

 but since Pittman's (1931) description of a " smooth " capsulated form of virulent 

 H. influenzce it is now clear that capsulation is not an uncommon feature within 

 the group. 



