PART SECOND, 
J 
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, asin 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. Itis 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. 
Sjébring 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- 
