36 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



at the expense of that of the cell envelopes. ExperimentaUy, it can be shown 

 that DNA of extrinsic origin may become adsorbed upon the bacterial cell 

 envelopes in the form of granules of very deceptive appearance. The 

 possibility of a transfer of nucleic acids between the nucleus and cell envelopes 

 must always be taken into consideration whenever small, basophilic structures 

 are under examination. 



Septa of a different and specialised type, with an active basophilic component 

 are found in the mother-cells of Rhizohiimi swarmers. These fundamentally 

 resemble the polysaccharide-complex cross-walls of Bacillus species and are 

 lined on both sides with basophilic material resembling the normal, membran- 

 ous septa, but greatly thickened. They appear to provide a secretory lining 

 to the lumen of the cells wherein the tiny swarmers are formed. The mis- 

 conceptions which have arisen from the appearance of these " barred cells " 

 are discussed in a later section : (Chapter VII). 



A recent and very important study on the difterentiation of the cell wall, 

 membrane and cytoplasm by Robinow and Murray (1953), should be read 

 in the original by all who are interested in this subject. 



C; DEl'ELOPMENT OF THE CELL ENl'ELOPES 



(3, Ch 7, f<, 9, IS, i'"^, 19, ^9, 30, 64, 71, 72, Si, S6, S7, 92, 93, 94. 9S, 96, 97. 9^) 



The fundamentals of bacterial cell division described by Schaudinn (1902) 

 1903) have been well substantiated by later work. The two main morpho- 

 logical types of bacteria, which correspond to the " smooth " and " rough " 

 colony forms, divide by constriction of the cell wall and by the formation of 

 cross-walls respectively. In the " rough " type, which is especially typical of 

 the large. Gram-positive bacilli, the division of the cells by cross-walls proceeds 

 more rapidly than does their complete separation, so that the coiled bands of 

 filaments are formed which give the well-known " Medusa-head " colony 

 appearance. The actual separation of r(uig]i bacilli docs ncn necessarily occur 

 immediately after the completion of cell division. Nor, wlien it does occur, is 

 it invariably the most longstanding cross wall which is split, although this 

 is usuallv the case. Separation may occur in such a manner as regularly to 



