4 Lectures on Bacteria. [$ i. 



Perseverance and constantly improving methods of research 

 advance our knowledge as time goes on. 



The Bacterium-cell is mainly composed of a portion of 

 protoplasm, which in the smaller and in most also of the 

 larger forms appears as an entirely homogeneous translucent 

 substance, but in some of the larger forms it is also often finely 

 granular or shows a different kind of structure, which will be 

 further described presently. It consists, as Nencki (2) has shown 

 in a number of cases, chiefly of peculiar albuminoid compounds 

 (mycoprotein, anthrax-protein) which vary with the species, and 

 its behaviour, when the usual empirical reagents are applied to it, 

 agrees in general with that of the protoplasmic bodies of other 

 organisms the yellow and brownish-yellow coloration with 

 solutions of iodine, and the absorption of, that is to say, the 

 intense staining by, preparations of carmine and anilin dyes. 

 Yarious specific differences occur in individual cases in the 

 behaviour of the protoplasm to these colouring reagents, and 

 supply very useful marks of distinction in certain cases which 

 will be mentioned again on subsequent occasions. 



We have already alluded to the fact that the protoplasm of 

 certain Bacteria described by Engelmann and van Tieghem, 

 for example, Bacillus virens, v. T., is coloured by chlorophyll, 

 being of a uniform pale leaf-green hue. In the very large 

 majority of cases it is colourless ; most Bacteria, not only when 

 isolated under the microscope but also when collected into 

 masses, have a pure or dirty-white colour, and in the latter case 

 show various shades of tint inclining to gray or yellow, &c., 

 which the practised observer may even apply to the determina- 

 tion of species. On the other hand, there are not a few Bacteria 

 which exhibit lively colours when they are associated in masses, 

 yellow, red, green, violet, blue, brown, &c., according to the 

 individual. Schroter has collected together a number of such 

 cases. How far these colours belong to the protoplasm itself 

 or to its envelope, the cell-membrane, which will be described 

 presently, or to both, cannot in most cases be certainly ascer- 



