136 VERMONT AGRICULTURAL REPORT. 



incubator is filled with water. The incubator is kept at a con- 

 stant temperature of about 98 F. After a week to ten days 

 the tubercle bacilli will be seen spreading out in all directions 

 from the particle with which the flask was inoculated, and final- 

 ly the surface of the liquid will be covered with a layer of 

 tubercle bacilli. When this is noted, the flask should be care- 

 fully shaken, so as to cause most of the growth upon the sur- 

 face to be immersed in the culture liquid and to sink to the 

 bottom of the flask. A small particle, however, should be left 

 on the surface to serve as seed for a new surface growth. This 

 shaking down of the surface growth can be readily accomplished 

 by rotating the flask two or three times very gently, and after 

 a little practice it will be found to be an easy matter to preserve 

 the desired particle upon the surface. From this particle a new 

 surface growth is developed, which should be shaken down as 

 in the first instance, and a third growth allowed to form. This 

 process will require six weeks to two months or more from the 

 time that the flasks were first inoculated, and their contents are 

 then in a condition to be further used for the preparation of tuber- 

 culin. 



When first obtained from the animal body, the tubercle 

 bacillus grows best upon blood serum, or potato, or a liquid, 

 such as has been already indicated, that has a faint alkaline 

 reaction to litmus or is perfectly neutral. After a time, how- 

 ever, when the tubercle germ has become accustomed to its new 

 food, just as a plant must adapt itself to a new soil, it can be 

 caused to grow upon medium that has a slight acid reaction. 

 When liquid cultures of the tubercle bacilli have been once in- 

 oculated and are growing well, it is very much easier to inocu- 

 late fresh culture media from liquid cultures rather than from 

 the jelly cultures, with which it is always necessary to start. 

 The transference of a particle containing large numbers of the 

 germs from the surface of one flask to serve as seed upon the 

 surface of another flask will give what is commonly called a 

 new generation. As these transfers are usually made every 

 month or six weeks, it is possible in the course of a few years, 

 to obtain a germ which is a direct descendant of the one orig- 

 inal used, but removed from it by many generations. This 

 continued transference of the bacteria from one nutrient flask 

 to another has the effect in many cases of changing some of 

 the properties of the germ. In the laboratory of the Biochemic 

 Division it has been found, in connection with the tubercle 

 germ, that this fact can be utilized to great advantage. There 

 are in the laboratory now, and have been for a number of 

 years, the descendants of a tubercle germ which originally 

 caused the death of guinea pigs in from four to five weeks 

 after they had been inoculated. This germ, which is now per- 



