6 TEXT-BOOK OF BACTERIOLOGY. 



toplasm, which is inclosed in a well-defined limiting' membrane, or 

 cell wall. But in regard to the existence of a nucleus there seems 

 to be some doubt. 



Until lately we were inclined to deny this in toto. It was shown 

 in the examination of unaltered bacteria that nothing could be seen 

 of a nucleus, even with the best optical aids; but under the influ- 

 ence of those coloring agents known as nuclei stains, the whole 

 individual bacterium became evenly and equally colored, and the 

 reaction which is otherwise seen in the nucleus alone extended here 

 to the entire cell contents. 



But every one of experience in these matters will see that this 

 does not form a rule without exceptions. Occasionally, under still 

 tmknown conditions, especially, however, after a short exposure to 

 active coloring agents, one sees how a part only of the bacterial 

 body greedily sucks up the coloring matter, while the surrounding- 

 parts remain paler and show clearly in contrast with the centre. 

 Such pictures, which cannot be explained as artificial productions 

 or faults of preparation, do indeed lead us to suppose that we have 

 a nucleus before us, with its protoplasmic body, and that the pres- 

 ence of the former generally escapes observation only because our 

 method of observation is imperfect and unable to show such fine 

 differences. 



This supposition becomes the more probable, as uncolored bac- 

 teria have been recently said to show something similar. 



We must of course be very careful to avoid self-deception. I 

 by no means wish to say that the cases mentioned proceed from an 

 error. I believe, on the contrary, that these observations are cor- 

 rect, yet a convincing proof for my opinion I cannot give, and it 

 will require further and deeper investigation to bring the question 

 to a final settlement. 



The contents are generally seen as a homogeneous, translucent, 

 dull mass, without traces of anj' particular network. 



But now and then we do see a sort of granulation, and such 

 molecular thickenings of the protoplasm may for a moment deceive 

 us into the belief that we see a structure. 



Some few bacteria possess chlorophyl in their cell contents; 

 others present a peculiar reaction, reminding one of the similar be- 

 havior of granulose under the action of an aqueous solution of iodine : 

 when treated with the same, thej' take a deep indigo-blue stain. 



The membrane consists, perhaps, of a mass similar to cellulose 

 belonging to the members of the carbo-hydrate group. It (the 

 membrane) is hard to recognize under the microscope without 

 further preparation, but if we employ measures which cause a con- 



