RHODO-THIOBACTERIA 155 



membrane. The colour varies from a deep violet to mauve. 

 Sometimes it is perceptible in single cells, but usually only when 

 they appear in groups. The separation of the cytoplasm from 

 the membrane can readily be brought about by the use of 

 concentrated reagents. 



The sulphur granules may be few in number, or they may 

 completely fill the cell ; they may even be altogether absent, 

 even in specimens that appear to be perfectly healthy. 



Fig. 34. — Thioporpliyra voliitans (Ellis). 



a. — Spherical cell with reticulate plasma (diagrammatic), showing uniform 

 distribution of vacuoles. X 6000. 



b. — Ovoid organism in process of division. Cell is constricted in the middle 

 preparatory to division. In the cell are four sulphur globules and two 

 globules from which the peripherally placed sulphur has been removed 

 by reagents. This core is organic. X 4000. ' 



c. — An ovoid cell in which the plasma is almost entirely disposed in the 

 form of a cap covering about three-quarters of the periphery of the cell 

 inside the membrane. Extensions from this cap to cover the rest of 

 the membrane on its inner side with an extremely delicate lining may 

 be demonstrated by special staining. The centre of the cell is 

 occupied by a large vacuole and at one end a large number of sulphur 

 granules are congregated. X 4000. 



d. — An ovoid cell showing a third mode of distribution of plasma. The 

 centre of the cell is occupied by a large vacuole. The plasma is more 

 or less evenly distributed between the vacuole and the membrane. 

 X 6000. 



e. — Sulphur crystals obtained by dissolving the sulphur globules with 

 aceto-carmine when they leave the cell and crystallize in the surround- 

 ing medium. 



/.• — Cell showing its single long and wavy ciliimi. 



g. — After treatment with aceto-carmine. Some of the sulphur globules 

 remain unaffected, in others the sulphur has disappeared, leaving a 

 coloured centrum. One sulphur crystal is shown, x 1000. 



The membrane is readily seen without the use of stains. 

 As with all bacterial cells a layer of slime covers the membrane 

 at all times. Under some conditions the layer is too slight 

 to be perceptible without very careful staining. Under other 

 conditions slime formation is extensive and a thick layer covers 

 the membrane. 



The cytology of the cell is treated with greater detail in 

 Chap. X. 



