436 BACTERIOLOGY. 



lines drawn about 1 or 2 cm. apart. The lines may be 

 drawn with pen and ink. They need not be exactly 

 the same distance apart, nor exactly straight. Beginning 

 with one of these squares at one end of the tube, which 



Fig. 90. 



Esraarch's apparatus for counting colonies in roll tubes. 



may be marked with a cross, the tube is twisted with 

 the fingers, always in one direction, and the exact 

 number of colonies in each square as it appears in rota- 

 tion is counted, care being taken not to count a square 

 more than once ; the sums are then added together, and 

 the result gives the number of colonies in the tube. This 

 method may be facilitated by the use of a hand-lens. 



In all these methods there is one error that is difficult 

 to eliminate : it is assumed that each colony represents 

 the outgroAvth from a single organism. This is prob- 

 ably not always the case, as there may exist clumps of 

 bacteria which represent hundreds or even thousands of 

 individuals, but which still give rise to but a single 



