§ IV.] TAe question of species. 35 



organisms have to be isolated and unceasingly watched in order 

 to see what proceeds from the single individual, if it developes. 

 This end can only be obtained by means of cultures which can 

 be followed with exactness under the microscope. A spore or 

 rod in the preparation must be permanently iixed under the 

 microscope, and the phenomena of its growth must be observed 

 without interruption. This is done by help of the moist 

 chamber, a contrivance in which the microscopic object pro- 

 tected from desiccation can be observed continuously under con- 

 ditions favourable to vegetation. There are several varieties of 

 apparatus of this description, which have their advantages and 

 disadvantages according to the special case and also to the 

 habits of the observer, but we must not enter into a detailed 

 description of them here. 



Fluids are usually employed as the medium in which the ob- 

 ject is placed for microscopic observation and for culture, on 

 account of their transparency. Living and especially moving 

 objects readily change their position in a fluid and become 

 mixed together. A method which greatly assists the fixing of 

 an object where continuity of observation is required, consists in 

 the use of a transparent medium which allows of the conditions 

 necessary to vegetation, and is soft but not fluid, so that dis- 

 placement of the objects and disturbance of the observation are 

 more or less perfectly avoided. Such media are gelatine and 

 similar substances, especially the gelatinous substance known 

 in commerce as agar-agar and prepared from sea-weeds of 

 the Indian and Chinese seas. Gelatine, as I understand, was 

 first employed by Vittadini in 1852 in the culture of micro-' 

 scopic Fungi (11), and has been frequently used since that time, 

 especially by Brefeld. Klebs more recently in 1873 (^2) re- 

 commends it specially for the cultivation of Bacteria ; cultures 

 of these organisms have been conducted in recent times in a 

 gelatinous substratum, especially by Koch. 



Having thus glanced at the morphology and the history of the 

 development of Bacteria, we have still to enquire what is their 



D 2 



