PLEUROCOCCUS. 85 



is frequently to be observed, which is doubtless the 

 result of the tendency of the cells to assume ( a 

 spherical form. 



Select a single large cell for examination under the 

 highest power, and observe the following details 



1. The cell- wall is thick, and shows clearly a double 

 contour : staining with Schulze's solution gives a blue 

 coloration of the cell-wall. 



2. The protoplasmic contents, which at first sight 

 appear uniformly green, will show differentiation, 

 especially after staining with iodine solution : thus 

 there may be recognised 



a. A nucleus, which is usually central, though 

 sometimes it is in a lateral position. 



6. Chromatophores, a number of which together 

 form a partial envelope surrounding the nucleus : 

 there are no pyrenoids. 



c. A very scanty colourless protoplasmic basis, in 

 which these bodies are embedded. 



It is held that Pleurococcus is the resting stage of Chlamydo- 

 monas, an organism which differs from other Volvocineae in that 

 its motile cells are separate, and do not form a coenobium. 

 Material of Pleurococcus should be kept in water exposed to 

 light, and observations made from time to time to see the motile 

 cells, with their limiting cell-wall and active cilia : the latter are 

 best seen after staining with iodine solution. 



Hydrodictyon utriculatum, Roth. 



Observations may with advantage be made on Hydrodictyon, 

 because of the very peculiar formation of the tissue composing its 

 network. It is not, however, of very frequent occurrence : it is 



