BACTERIA. 



49 



portion of the cell wall. This material is mucin-like and is soluble in 

 water. The bacteria which grow in the bodies of animals frequently con- 

 tain these capsules but fail to show them when grown upon artificial cul- 

 ture media. It is difficult, therefore, to determine whether or not an 

 organism has a capsule by mere examination of cultures. Some culture 

 media, however, do cause a formation of capsules in the case of capsulated 

 bacteria. These are blood serum, sometimes, and milk, usually. Beau- 

 tiful capsules can be obtained by growing such bacteria as the Bact. 

 pneumonia, Bact. capsulatum, and Bact. welchii in milk cultures. Strept. 



FIG. 29. Capsules. 



Bact. pneumonia (Fried lander). 

 from Frost and McCampbell.) 



(After Weichselbaum 



mesenteroides is a bacterium which grows in the syrup of the sugar re- 

 fineries and forms abundant capsules. This organism changes the char- 

 acter of the syrup, and its entrance and growth is frequently the cause of 

 serious loss. 



Sheath. Among the higher bacteria, such as Crenothrix, there is 

 present a thickened and hardened membrane which is spoken of as a 

 sheath. It forms a tube in which the different cells of the plant are con- 

 tained. This sheath is homologous to the capsule and in it are frequently 

 deposited certain by-products of the cell. In Crenothrix we frequently 

 have iron oxides (p. 55). 



BACTERIAL CYTOPLASMS. The cytoplasm of the bacterial cell is 

 similar to the cytoplasm of other cells except that chemical analyses seem 

 to show that it contains a higher percentage of nitrogen. As viewed under 



