82 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



In myxobactcria and cubactcria the rod form of die nucleus divides into 

 two halves which fuse again into a central nucleus. In some cases the con- 

 jugation appears to be autogamous, the two daughter nuclei recombining 

 within the original cell. In others the cell divides completely and the two 

 halves become gametes. 



In rough, sporing bacilli the nuclear units of the four cells combine to 

 form a single, rod-shaped fusion nucleus, from which, after a reduction 

 process, the spore nucleus is derived. In sporing baciUi of smooth morphology 

 the fusion nucleus is formed from the two nuclear units of the cell, and the 

 reduction process differs in detail from that of the rough types. 



In streptomyces all the cells oi the spore-bearing, secondary mycelium 

 contain similar fusion nuclei, derived by an unusual type of conjugation from 

 the primary mycelium. 



In M. tuberculosis the vesicular nucleus arises directly trom the fusion of 

 two of the small, spherical nuclei. 



The details ot these sexual processes will be described in Chapter VI. 



K: SUMMARY 



In resting cells the bacterial nucleus is a spherical or vesicular structure, 

 lying centrally in the cell but often staining eccentrically. It is foimd in this 

 form in spores, microcysts and the resting stages o( most bacteria. A similar 

 type of nucleus is found in the active stages of certain cocci, mycobacteria 

 and other bacteria of similar morphology. 



More frequently the vegetative nucleus is in the form of paired chromo- 

 somes or chromosome complexes. These are short rods lying transversely 

 to the long axis of the cell. The chromosomes take part in simple vegetative 

 division, in which they split longitudinally, dividing with the cell, and in a 

 complex, possibly sexual, method ot division, in which a nuclear hision is 

 followed by elongation of the bacteriimi as a filament, redistribution oi the 

 nucleus and finally fragmentation to produce a new generation of bacilh. 



The distribution of the nuclear material at cell division does not follow 



