108 SCIENCE PROGRESS. 



and even exaggerated involution forms which can under 

 favourable conditions be seen in twenty-four hours if cultures 

 are made on agar which contains 2.5-3.5 P er cent °f sa ^- 

 If kept at a temperature of 37° C. extraordinary involution 

 forms are met with. Large spheres and pear-shaped bodies 

 which often have rod-like extensions are seen, but at later 

 periods of growth these very abnormal forms are not fre- 

 quently met with. For the success of this method as a test 

 for the bacillus it is necessary to first cultivate the microbe 

 on agar and then transfer it to the salted medium. Haff- 

 kine had previously recommended the use of agar having 

 a well-marked alkaline reaction for the rapid production ot 

 involution forms, but since it is not possible to keep this 

 medium long at the required degree of dryness, or to obtain 

 such certain and quick results, it is not to be preferred to 

 a salt-agar medium. 



The pest-bacillus is easily cultivated outside the body, 

 and indeed grows well on all varieties of media. One of 

 the best of these is the one originally recommended by 

 Yersin, whose statement is confirmed by Wilm (12). It 

 consists of 2 per cent, alkaline peptone solution with the 

 addition of 1-2 per cent, of gelatine. Both on this and in 

 ordinary bouillon made from horse-flesh a culture is obtained 

 which much resembles that of streptococcus pyogenes ; 

 flocculent deposits forming at the bottom and sides of the 

 tube, while the rest of the liquid remains clear. At times 

 this liquid becomes diffusely turbid, and these two types of 

 growth in bouillon are produced, according to Abel (13), 

 who worked with a culture derived from Kitasato's orio;inal 

 stock and one obtained from a fatal case of plague in 

 London during October, 1896, by the manner in which the 

 medium is inoculated. Neither on Loffler's blood-serum 

 on coagulated ascitic fluid, on gelatine, which is never 

 liquefied by the growth, on glycerine-agar, nor on agar-agar, 

 does the bacillus thrive so well as in bouillon. On the last 

 of the solid media a maximum rate of growth is reached in 

 twenty-four to forty-eight hours at the body temperature, 

 and isolated white transparent colonies with iridescent edges 

 are formed. These colonies differ in size and rate of growth, 



