40 CELL STRUCTURES 



occasionally happens that water diffuses into the cell ^nd 

 swells up the endoplasm so much more rapidly than the cell 

 wall that the latter ruptures and some of the endoplasm 

 exudes in the form of droplets on the surface of the cell 

 wall. This phenomenon is called plasmoptysis. Students will 

 seldom observe the distinction between cell wall and cell 

 contents, except that in examining living bacteria the outer 

 portion appears more highly refractive. This is chiefly due 

 to the presence of a cell wall, but is not a proof of its 

 existence. 



Fig. 16. — Cells of bacteria show- Fig. 17. — Vacuoles in the bac- 



ing plasmolysis. The cell substance terial cell. The lighter areas are 



of three of the cells in the middle of vacuoles, 



the chain has shrunk until it appears 

 as a round black mass. The cell wall 

 shows as the lighter area. 



Nucleus.^ — A nucleus as such is not present in bacterial 

 cells, except in a few large rare forms. Nuclein, the char- 

 acteristic chemical substance in nuclei, which when aggre- 

 gated forms the nucleus, is scattered throughout the cell con- 

 tents and thus intimately mingled with the protoplasm, and 

 cannot be differentiated by staining as in most cells. This 

 close association of nuclein and protoplasm may explain 

 the rapid rate of division of bacteria (Chapter VIII, p. 80). 



The chemical composition of the bacterial cell is discussed 

 in Chapter VII. 



In addition to the essential parts just described the bac- 

 terial cell may show some of the following accidental struc- 

 tures : vacitoles, capsules, metachromatic grannies, flagella, spores. 



