BLOOD-SERUM. 69 



to be cut a slanting surface running diagonally from about 

 the junction of the first and second third of the cylinder 

 to the diagonally opposite end. These cylinders of potato 

 are now to be left in running water over night, otherwise 

 they are very much discolored by the sterilization to 

 which they are to be subjected. At the end of this time 

 they are placed into previously prepared test-tubes, one 

 piece in each tube, with the slanting surface up, the cot- 

 ton plugs of the tubes replaced and they are then to be 

 sterilized in the steam for forty-five minutes. On the 

 second or third day they are to be sterilized 

 for fifteen to twenty minutes each day. FIG. 6. 



The entire sterilization may be accom- 

 plished in the autoclav with the steam under 

 a pressure of one atmosphere, by a single 

 exposure of twenty to twenty-five minutes. 

 When finished they have the appearance seen 

 in Fig. 6. 



Care must be given to the steriliza- 

 tion of potatoes, because they always have 

 adhering to them the organisms commonly 

 found in the ground, the spores of which are 

 among the most resistant of all known or- 

 ganisms. The so-called " potato bacillus " is 

 one of this group ; it is an organism which not 

 infrequently is more or less of an obstacle 

 to the work of the beginner. 



BLOOD-SERUM. Blood -serum requires 

 special care in its preparation ; it is desir- 

 able under all conditions to reduce the unavoidable 

 contamination which to a certain extent occurs during 

 the manipulation, to its minimum degree. 



It is possible to collect serum from small animals and 



