DISTINCTIVE CHARACTERS OF THE BACTERIA. 109 



line, or whether it spreads more or less rapidly from 

 it towards the periphery ; whether the spreading margin 

 is straight or irregular, sinuous or jagged, &c. 



In the case of many organisms which liquefy gela- 

 tine energetically, we may now and then employ with 

 advantage a mixture of the nutritive solution with agar- 

 agar, which remains solid at 37 C. ; nevertheless this 

 cultivating medium does not on the whole show such 

 characteristic differences in growth. Some bacteria 



Fig. 42. Stroke cultivations on gelatine. 



grow only on solidified blood serum at the body tempera- 

 ture, and may form very characteristic colonies ; finally, 

 some only grow to visible colonies when free oxygen 

 is excluded, and can therefore only be cultivated with 

 special precautions, to be afterwards described. 



4. Classification and Distinctive Characters of the 

 Bacteria. 



A systematic classification of the bacteria is a matter 

 of very great difficulty, because they are so minute 

 that even by the use of the highest powers of the 

 microscope only the external form, and not the differ- 



