SCIENCE BULLETIN, No. 21. 



invader, and with such attenuated organisms the dose administered was too 

 small. That the bacilli do become attenuated in this manner has been 

 demonstrated, even when fresh sub-cultures are employed for inoculation. 

 More detailed experimental work showed pretty conclusively that the bacillus 

 in question could not be regarded merely as a harmless secondary invader of 

 tissues already damaged, for such work demonstrated that small amounts of" 

 virulent cultures of the organism are capable of bringing about a speedily 

 fatal result upon par-enteral inoculation, and that the lesions thus artificially 

 produced by it in the sheep are anatomically those of black disease. 



It remains to be stated that the foregoing views are not hasty conclusions 

 based upon insufficient evidence, but are the outcome of the examination of, 

 and experimental work with, tissues from a large number of sheep dead of 

 black disease, and as controls, sheep naturally affected with various other 

 conditions. 



The Causal Organism of Black Disease. 



Morphology. The organism is one of the larger sporing bacilli. Its 

 length varies considerably, depending on the element on or in which it has 

 been growing. Its breadth is, however, fairly constant. In the natural 

 hepatic lesions its lengtji is from about 4 to 8 microns, with numerous 

 filamentous elements up to 60 microns, without apparent division. The 

 length of the majority of these filamentous forms is, however, about 

 30 microns. Occasionally chains composed of three or four shorter elements 

 are encountered, but single rods are in the majority. The ends are 

 distinctly rounded. The breadth is about 0-8 to 1 micron. Scattered 

 among the before-described forms may be seen here and there short bacilli,, 

 about 2 microns long, and also even what at first sight appear to be coccus- 

 like elements. These latter, as already stated, are merely the ends of the 

 longer rods, standing at right angles in the section. The short rods are the 

 ordinary bacilli lying obliquely to the line of vision in the section, as may 

 be readily demonstrated by altering the focus of the microscope, the rest of 

 the rod then coming into view, whilst the former part disappears. The 

 filamentous elements are almost always to be found arranged around the 

 periphery of the lesion often in masses near the leucocytic zone. The 

 bacilli scattered throughout the body of the lesion are of the shorter type. 



In cultures the bacilli usually occur singly or in pairs. No chains are 

 formed, and only an occasional filamentous element is seen. In liquid media 

 their length is from about 4 to 7 microns, and their breadth from 

 0'8 to 1'2 microns. Occasionally rods up to 10 microns occur, but they 

 are seldom less than 4 microns long. In primary cultures from the 

 muscle of inoculated sheep in serum-formate broth, elements up to 

 17 microns long are frequently seen. Involution forms often occur when 

 the media or surroundings are unfavourable. In smears taken from the 

 local lesion of experimentally infected animals the bacilli vary from 2-5 to 

 7 microns in length, but their breadth, as in other cases, remains fairly 

 constant, viz., about 1 micron. The longer bacillary forms predominate, 

 but there are no chains or filamentous elements. The organism is non- 

 motile. 



