SURFACE STRUCTURES 47 



/. SUMMAR Y 



Bacteria possess a cell wall oi great strength and rigiditv, overlving, and 

 secreted, in certain well-marked areas, by the underlying, senii-pernieable 

 cell membrane. Bacteria may be subdivided by cross-walls into a number of 

 cells varying from two to twenty or more. In unicellular bacteria the main 

 growth of the cell wall is at the tips ; in septate forms, the main growth is at 

 the junction of cell wall and cross-walls. These sub-divisions and this mode 

 of growth extends also to the capsule, which may be verv complex, with 

 polysaccharide and polypeptide lamina. 



The cell wall contains polysaccharide, lipid and protein elements. The 

 chemical composition of the cell wall is more complex in Gram-negative 

 than in Gram-positive bacteria. The cell membrane contains lipid, protein 

 and nucleo-protein elements. It gives rise to the flagella. 



In cell division a septum derived from the cell membrane precedes and 

 secretes the cross-wall or ingrowth of the cell wall. These septa and the 

 growing points which secrete the cell wall are basophilic and physiologically 

 active. There is evidence that they constitute a major somatic antigen. They 



Figs. 16 and 17 



DEVELOPMENT OF FLAGELLA 



Development of flagella in the germinating microcyst. The resting cells of flagellated 



bacteria are devoid of flagella ; on germination these develop first at the poles of the cell, 



especially at the pole remote from the growing point. Electron micrographs, gold-shadowed. 



(1, 2, 3, 4) Salmonella typhi, stages in the germination of the microcyst. 



(I) Microcyst, without flagella. x 30,000. 



(2, 4) Young vegetative cells with short flagella concentrated towards one pole of the 

 cell. ■ 9,000 and x 7,000. 



(3) Germinating microcyst with very short flagella towards both poles. ■ 27,000. 



(5, 6, 7) Bacfeniim coll, stages in the germination of the microcyst. 



(5) Germinating microcyst with two very short flagella originating at one pole of the cell 

 from an obvious blepharoplast. > 20,000. 



(6, 7) Two polar flagella further developed. ■ 16,000. 



(8, 9, 10) Development of flagella demonstrated by silver impregnation stain. 3,000. 



(8) Proteus, germinating microcyst. 



(9, 10) Sal. typhi, very young cells w^ith sub-polar flagella. 



(II) Germinating microcysts of Pseudomonas fluorescens. The flagella emerge more 

 closely together than in the case of the foregoing examples, which will e\entually become 



16,000. 



