CULTURAL CHARACTERISTICS 543 



with a low-power objective, places will be found where the ameba 

 are some distance apart. After locating a satisfactory parasite, 

 which should be one on the surface of the medium, as practically all 

 of them are, and having determined that there are no others in the 

 field, either on the surface or at a depth, swing a perfectly dry and 

 clean high-power lens in place and gently lower it until the entire 

 surface is in contact with the medium. Raise the lens quickly, swing 

 in the low-power objective and determine whether the ameba is 

 still present or has been picked up. If it has been picked up, which 

 after some practice may be done two or three times out of five, the 

 lens to which the ameba adheres is removed, and, by gently rubbing 

 its surface over that of a plate containing the hardened medium the 

 organism may be transferred. In this manner a pure culture, so 

 far as amebae are concerned, may be obtained. That only one 

 ameba has been carried over by this method may still farther be 

 verified by examining with a low-power objective the closed inverted 

 plate on which it has been inoculated. Another useful result of a 

 careful application of this method is the aid it gives in obtaining pure 

 cultures of an ameba and of a single bacterium. The lens, of course, 

 picks up the bacteria from a small field immediately surrounding 

 the ameba; and as such isolated ameba is often surrounded by one 

 kind of bacterium only, with the aid of a careful bacteriologic tech- 

 nique the pure cultures desired may sometimes be obtained in this 

 manner." Musgrave and Clegg have used the preceding method in 

 most cases and have found it satisfactory. It would be possible to 

 obtain the desired results more easily and with greater constancy 

 by means of Unna's bacterial harpoon or a specially constructed 

 lens, with a short adjustable focus and a cup-shaped extremity, like 

 the marking arrangement which has been suggested for locating 

 special fields in permanent preparations. 



Parasitic amebse obtained in cultures on plates growing there in 

 symbiosis with bacteria will not grow with any and all bacteria, 

 but they are quite selective, at least, at first, and it is, therefore, 

 necessary to obtain from the mixed culture on the first plates all 

 bacteria present in pure cultures. After these have been obtained 

 and a number of individual amebse have been picked out by the 

 method described above and transferred to fresh plates such planted 

 amebse are surrounded by several concentric streaks of bacteria 

 obtained from a pure culture. If the necessary precautions have been 

 taken, most amebse, as they multiply, will quite generally spread 

 rapidly over the plate, and in passing through the rings of growing 

 bacteria they will lose the organisms with which they started and 

 take up those forming the rings. In from twenty-four to seventy- two 

 hours the protozoa will have passed one or more of the rings, and 

 from such locations they may be taken for transplanting. It sometimes 

 happens that they appear on the first plate in pure cultures with the 

 desired organism, but usually one or more transplants to the same 



