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 Bab6s 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 



