MOUNTING OF PERMANENT PREPARATIONS. 145 



distinguish clearly any minute points in their struc- 

 ture. 



Nevertheless a good collection of bacteria prepara- 

 tions is of great use. A large number of species may 

 be quickly glanced over, in order, for example, to 

 compare them, and species may be examined, of which 

 perhaps no cultures are just then to be obtained. 

 Very often too, interesting stages of development or 

 peculiarities of growth or differences in position are 

 preserved, such as probably will not readily be ob- 

 tained again ; and finally, by means of a good collection 

 of such preparations, the student is able to recapitulate 

 quickly the important points in the life history, the 

 development, and other peculiarities of a certain species. 



The collection, however, is really only of great use 

 if it is systematically arranged. Above all things, the 

 student should beware of saving imperfect prepara- 

 tions and of trying by their great number to atone for 

 their imperfections. None but absolutely perfect 

 preparations are worth preserving. Further it is best 

 at first to aim at getting several specimens of a few 

 kinds in different conditions and stages of develop- 

 ment, rather than to collect as many different kinds 

 as possible. For instance, the hay bacillus affords a 

 large amount of suitable material. There is great 

 variety in the vegetative condition alone, although 

 within narrow limits ; these bacilli occur in the form 

 of long threads, of single cells, either motile or motion- 



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