Organisms Consisting of One Cell 257 



earliest unicellular organisms can only have been evolved 

 from the simplest organisms we know, the monera. Tliese 

 are the simj^lest living things that we can conceive. Their 

 whole body is nothing but a particle of plasm, a granule of 

 living albuminous matter." -' So far as the protozoa are 

 concerned, no zoologist who is both well inforaied and in- 

 tellectually free and candid pretends any longer that such 

 beings are known. "In all protozoa that have been exam- 

 ined in recent times, at least one nucleus has been found 

 to occur without exception." ^^ It hardly needs to be re- 

 marked that even the possession of a nucleus is sufficient to 

 take an organism out of the category "organisms without 

 organs." But as a matter of fact the nucleus is not the 

 only differentiated part that has been demonstrated in the 

 simplest protozoa. 



The Structure of Bacteria 



The visible living beings to which the term structure- 

 lessness can be ascribed with the greatest plausibility are 

 the heterogeneous myriads known as the bacteria. Whether 

 or not these organisms are ti*ue cells, that is, are "nucleated 

 masses of protoplasm," has been extensively debated in re- 

 cent years. This much may be regarded as settled: If by 

 nucleus one is to understand a cell organ of such structure 

 as that which is characteristic of ordinary ])lants and ani- 

 mals, then the bacteria are not nucleated. But this is far 

 from meaning that the organisms are structureless. 



(a) Membrane and Surface Structures 



In the first place, there seems to be almost complete agree- 

 ment among authorities that in bacteria the body is differ- 

 entiated into at least one outer coat or membrane and an 

 inner mass. It should be specially noted that the membrane 

 is by no means a mere passive, wholly extrinsic thing, like 



