METHODS OF INVESTIGATING BACTERIA. 797 



lation, we have a guarantee for the purity of the second 

 cultivation. 



Among these solid nutrient substrata potatoes are Transparent 



. ,, . -v T . , , .. . , , solid nutrient 



more especially convenient. JNevertneless, it is better substrata. 



if we can obtain a transparent soil, as, for example, by 



the admixture of gelatine or agar with the fluid ; on 



these transparent soils the majority of the bacteria grow 



in a very characteristic manner, and we can distinguish 



the colonies very easily, not only with the naked eye, 



but also by the aid of the microscope. I have already, 



on p. 167, pointed out the most important differences 



of the stroke and puncture cultivations on these nutrient 



substrata. 



It is, however, difficult to isolate the individual species isolation 

 of bacteria from a mixture by means of stroke and bactena - 

 puncture cultivations on solid nutrient substrata. If 

 after the gelatine has been allowed to solidify on a glass 

 slide we draw several long strokes over the surface with 

 the infected needle we observe that discrete colonies 

 appear at various parts of the track, and we can then re- 

 inoculate from these before they have coalesced with the 

 other colonies. The more we dilute the material to be 

 investigated, the more readily do we succeed in obtaining 

 these discrete colonies. But where numerous species 

 are present it may very readily happen that the 

 organisms are carried along the stroke, and that several 

 species develop at the same place. 



On the other hand, the organisms can be very readily Plate cnltiva- 

 isolated by means of plate cultivations. In this method tlons ' 

 the material to be investigated is in the first place 

 mixed with the fluid jelly in various degrees of dilution, 

 and this jelly, containing the subdivided germs, is then 

 poured out on a cold glass plate, on which it rapidly 

 solidifies. It is evident that in this method each of the 

 suspended germs will be fixed at a particular spot, and 

 if they are not too numerous, isolated colonies will 

 develop from each organism, and these can be recog- 

 nised by their characteristic appearance under the micro- 

 scope, and may be inoculated into test tubes. 



For this purpose we employ square, or, better, oblong 



