Morphological Studies in the Life- Histories of Bacteria. 477 



5. The same types of " aberrancy " were seen in every one of the different 

 organisms shown, as well as in all the strains of all the organisms not shown, 

 and of the different strains of the organisms, single strains of which are shown. 



In the accompanying photographs (Plates 16-19) and drawings attention 

 is called to the following points : — 



1. It is not claimed that a complete history of bacterial life-cycles can be 

 worked out in acid broth cultures. This can only be obtained by extensive 

 experimental observations, side by side with comprehensive morphological 

 studies of organisms as they occur in the infected tissues and body fluids of 

 subjects of disease. These, in the case of members of the enteric group, have 

 yet to be undertaken. 



2. In all the broth cultures studied, reproduction by simple binary fission 

 was still the predominant feature, and in studying the " aberrant " types of 

 reproduction of single living organisms on the warm stage on solid media, 

 such as gelatin-agar, ordinary binary transverse fission was found eventually 

 to hold the field mainly, though not absolutely, to the exclusion of other 

 types of reproduction. It appears from these observations from single living 

 individuals on the warm stage that reproduction by gemmation occurs freely, 

 in conjunction with ordinary binary fission, only so long as growth proceeds 

 in the thin layer of broth on the cover-slip, and largely comes to an end when 

 colonies are beginning to form on the solid medium. This abrupt transition 

 is well seen in Plate 20, as is also the familiar " slipping " phenomenon 

 described by Hill in 1904. 



3. The percentage population of " aberrant " forms in ordinary broth 

 cultures, +10 to phenolphthalein, was low, but the chief types of 

 " aberrancy " recorded could always be found if persistently searched for. 



4. The percentage population of " aberrant " forms in broth cultures 

 + 20 and + 60 to phenolphthalein, as well as in sub-cultures of these in 

 ordinary broth cultures, +10 to phenolphthalein, was high, each field of the 

 microscope yielding, in good films, characteristic types. 



5. Each photograph is designed to show at the optimum focus not more 

 than a very small number of types. 



6. The number of types which can be seen in any acid culture exceeded 

 100. To reproduce a comprehensive picture by photographs alone was 

 therefore impracticable. 



7. In consequence it became necessary to make composite drawings with 

 the camera lucida of the chief types observed in one or, at the most, two film 

 preparations from one strain. These drawings represent, in the case of 

 killed organisms, selected individuals from a large number of fields, and must 

 therefore not be read as representing average fields. 



