TEXT-BOOK OF BACTERIOLOGY. 313 



about as long as the tubercle bacilli, but twice as broad, hence 

 of a rather coarse appearance and usually' with rounded ends. 

 But the form of the micro-organisms is exceedingly variable and 

 the differences are striking in appearance. Really normal bacteria 

 of this variety are seldom met with. The bacteria are sometimes 

 seen enveloped in a more or less capacious, glassy membrane; 

 sometimes the contents separate into several pieces divided by a 

 broad, transverse wall; one end of the rods is very frequently 

 thickened like a club ; sometimes this change appears on both sides 

 so that there arise dumb-bell-shaped structures usually pointed out 

 as involution forms. The bacilli are immobile and liave no spores, 

 but perish rather rapidly when dried, and succumb at 45° to 50° C. 



They absorb the common anilin stains only imperfectly, but it 

 is not difflcult to prepare them with LofHer's alkaline methyl- blue. 

 The rod is evenly stained along its entire length. The terminal 

 pieces often prove more accessible to stain and stand out as dark 

 spots over the pale centre. Smaller roundish structures, so-called 

 " pole-granules," appear in the extremities and into these the color 

 enters most rapidly. 



The diphtheria bacilli are semi-anaerobic; they only thrive at 

 somewhat high temperatures (between 20° to 42° C). 



On plates of 15^ gelatin at 24° C, small, roundish, white colonies 

 of a moderate size are developed which never liquefy the medium. 

 They appear under the microscope as yellowish-brown, dense discs 

 of granulated structure with irregular edges. 



On plates of agar or glycerin-agar, at breeding heat, millet-seed 

 sized, flat colonies with a very flat border and of a grayish- white 

 lustre form in twenty-four to forty-eight hours; not rarely they 

 exhibit, macroscopically, a ring-shaped stratification. Under the 

 microscope they show pronounced similarity with the colonies of 

 the Bacillus megaterium on the gelatin plate — a gray mass of pe- 

 culiar coarse granulation, looking like " shagreen." 



In gelatin culture, small, white, round globules are formed along 

 the inoculation puncture. They grow but little at first. A phe- 

 nomenon is noticed on this medium which is still more distinctly 

 seen in cultures on oblique agar or glycerin-agar. With sterilized 

 scissors cut off a piece of diphtheritic membrane about the size of a 

 pin's head, and pass it with the platinum- needle successively through 

 six or eight tubes containing the culture medium; the quantity 

 of the germs distributed on the surface diminishes from tube to 

 tube; the fifth or sixth will exhibit distinctly separated, single col- 

 onies, some of which prove, on microscopic examination, to consist 

 of diphtheria bacilli. 



