42 BACTERIOLOGY. 
The conditions of temperature and of nutrition which 
favor growth are very various for different species, so 
that no fixed temperature, medium, or age of growth 
can be determined upon as applicable to all species. 
Morphological descriptions should always be accom- 
panied by a definite statement of the age of the growth, | 
the medium from which it was obtained, and the tem- 
perature at which it was developed. 
It is further advisable that the appearance observed 
in growths developed upon a solid and in a liquid 
medium should be recorded. 
The structure of bacterial cells has recently attracted. 
considerable attention among naturalists. According to 
Fischer and Migula, the bacterial cells consist of a cell- 
membrane, a protoplasmic layer, and a central fluid; no 
nucleus was observed by them. In salt solutions and 
when dried upon a cover-glass a shrinkage of the pro- 
toplasmic layer with partial dissolution of the cell-wall 
occurs, due to the abstraction of water. This process 
is known as plasmolysis, and it explains the occurrence 
of the clear, unstained spaces so frequently seen in 
the stained cover-glass preparations which have erro- 
neously been taken for spores. In water, or by 
the continued action of salt solution, this shrink- 
age does not take place. In many species of bacteria, 
such as the diphtheria bacilli, there is observed in the 
interior of the cells, on suitable staining, a peculiar 
granulation, to which Babés has given the name of 
metachromatic bodies, but which Ernst on more careful 
study has termed sporagenous granules. 
With regard to the cell membrane, it should be 
noticed that it is frequently not sharply defined and 
often difficult to demonstrate. In many species of 
