PART SECOND. 



I. 

 STRUCTURE, MOTIONS, REPRODUCTION. 



THE bacteria are unicellular vegetable organisms, and consist of 

 a cell membrane enclosing transparent and apparently structureless 

 protoplasm. The very varied biological characters which distin- 

 guish different species make it evident, however, that there are es- 

 sential differences in the living cell contents, although these differ- 

 ences are not revealed by our optical appliances. And among the 

 bacteria, as in the cells of higher plants and animals, the peculiar 

 biological characters of a species are transmitted to the cellular pro- 

 geny of each individual cell. These characters are, however, sub- 

 ject to various modifications as a result of differing conditions of 

 environment, as is the case with plants and animals higher in the 

 scale of existence, and in this way more or less permanent varieties 

 are produced. It is probable that among these lowly plants species 

 are evolved more quickly, as a result of the laws of natural selec- 

 tion, in the struggle for existence, than among those of more com- 

 plex organization. Still, this has not been proved, and, on the other 

 hand, we have ample evidence that widely distributed species exist 

 having very definite morphological and biological characters which 

 enable us to recognize them wherever found. 



It has generally been supposed that these simple vegetable cells 

 are destitute of a nucleus, but a recent author (Frankel) suggests 

 the probability that a nucleus may exist, although it has not been 

 demonstrated. This suggestion is based upon the fact that in stain- 

 ing bacteria very quickly it sometimes happens that a portion of the 

 protoplasm is sharply differentiated by taking the stain more deeply 

 than the remaining portion. 



Sjobring in 1892 made an investigation for the purpose of 

 ascertaining the structure of bacterial cells. Various methods 

 were employed, but the most satisfactory results were obtained by 

 fixing with nitric acid, with or without alcohol, and without pre- 



