78 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



rate of nuclear division in the latter stages of a culture, bringing about a 

 restoration of the condition found in very young cultures of smooth bacteria, 

 in which many cells contain only a single pair of chromosomes. Alternatively 

 it may represent a reduced or simplified condition of the nucleus, in preparation 

 for the formation of the resting nucleus. 



The secondary nuclear phase is usually tound in the latter stages of rough 

 cultures o( Bactenacea\ The filamentous type of growth may be preserved 

 but the cells become more elongated until eventually the unit bacilli are 

 unicellular, and similar to the secondary stage in smooth cultures. The 

 difference between the primary and secondary phases of the nucleus is less 

 in the case of rough than of smooth cultures, because the rough cultures 

 usually have only a single pair of chromosomes in each cell in the primary 

 phase also. 



Lactobacilli, however, which are normally of rough morphology, may 

 pass directly from the primary to the resting state of the nucleus. Each o{ 

 the four cells ot the vegetative bacillus is transformed into a microcyst. 



/; THE ROD-LIKE NUCLEUS 



(3, 13, 14, 20, 27, 30, 31, 32, 33, 36, 56, 57) 



The appearance ot the bacterial nucleus in the form ot a longitudinal, 

 chromatinic rod has frequently been reported, although considerable doubt 

 has been cast upon the significance of this appearance. Lewis (1941), in a 

 review of bacterial cytology, expressed the opinion that the central rod or 

 " spiral " nucleus was, at least partially, an optical illusion, due to the accumula- 

 tion of unstainable, reserve food materials, and especially fat globules, in the 

 cell. Lewis considered that this resulted in the compression of the remaining 

 stainable cytoplasm into a three-dimensional reticulum, the greatest thickness 

 of which, viewed through the middle of the cylindrical cell, gave the appear- 

 ance of an irregular, central, chromatinic rod. While it would be rash to 

 assert that this condition never arises, it is unquestionable that the bacterial 

 nucleus frequently assumes a rod form, and it is a regular part of those sexual 

 processes in bacteria which have been most fully described. Its form is often 

 such as to suggest that it is no more than the natural appearance of a mass of 



