TEXT-BOOK OF BACTERIOLOGY.' 109 



son is that the masses of bacteria in these pure cultures are usually 

 so great that many of the finer distinctions do not display them- 

 selves. Yet the experienced eye is, nevertheless, able to distin- 

 guish whether the culture is pure or not, and of what species it 

 consists^for the differences still remain suflftciently marked. 



In a series of cultures consisting of micro-organisms which do 

 not liquefy the gelatin, some are observed to grow equallj'- well all 

 along the puncture, either in thick, lumpy masses or in fine, deli- 

 cate granulations. Many, on the other hand, only develop on the 

 surface of the gelatin, while the puncture itself has remained un- 

 fruitful. This is a sign that the bacteria inoculated were such as 

 needed a plentiful supply of oxygen. 



Finally, some few kinds spread themselves throughout the en- 

 tire contents of the tube like misty clouds, which only become dis- 

 tinctlj' visible as fine clouds or veils when seen against a dark 

 background. Some color the gelatin in tints varying from a 

 bright purple-red to dirty gray ; others, again, array themselves in 

 plain white, and only become somewhat brown in old age. Numer- 

 ous bacteria liquefy the gelatin, and with them we can, as a rule, 

 only speak of plain distinctions in the beginning of their develop- 

 ment; afterward, when the liquefaction of the solid food medium 

 has become far advanced, the distinctive marks are lost. But at 

 first, as already said, we do see decided differences. Some grow and 

 decompose the gelatin rapidly, the puncture surrounds itself with a 

 "sock," or trouser-leg, of liquefied gelatin; others, which liquefy 

 more slowly, sink down from the surface, as it were in a funnel ; 

 some send out from the needle hole fine, bristle-like threads and 

 branches into the gelatin, and present quite an elegant appearance. 

 When the liquefaction has advanced still further, the motile 

 species proceed, as a rule, to the formation of a pellicle, a thick 

 superficial covering; the non-motile kinds sink gradually by their 

 own weight to the bottom, leaving above them a layer of perfectly 

 clear gelatin. 



If instead of piercing the food medium with the inoculating 

 needle we only draw it over the surface without entering the sub- 

 stance, we get a so-called needle-stroke culture. The needle-stroke 

 culture develops best in gelatin which has Hardened in a test-tube 

 plaeed obliquely. The surface at disposal is then much larger, and 

 the appearance of such an inoculating stroke is often very charac- 

 teristic. Of course this is only the case with the non-liquefying 

 bacteria : with the others the food medium soon sinks away from 

 the wall of the test-tube. 



Agar-agar, as we know, is free from the last-named fault. It 



