60 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



resulting from this reaction have been variously described as " extracyto- 

 plasmic," " peripheral " or " crcscentic " nuclei, but they are now generally 

 agreed to be artefacts. 



The identity of the spherical nucleus, as found in active cultures of some 

 genera, with that which is confined to the resting stages of most types of 

 bacteria, whose active nucleus is chromosomal, is not certain. Bacteria, 

 whose active nucleus is spherical but small, may possess a resting nucleus 

 which is larger and more obviously vesicular (Chapter VI). 



In the cells of most classes of living organism the nucleus returns from the 

 chromosomal to the resting condition between each division, but in bacteria 

 the mitotic condition of the vegetative cell may be retained throughout the 

 period of active reproduction, and the resting nucleus is restored only when 

 active reproduction ceases. The active condition of the nucleus is so much 

 more readily demonstrable that it has been supposed that the nuclear material 

 preserved an organised form only in yoimg cultures, and became disintegrated 

 and distributed throughout the cytoplasm when cultures were more than a 

 few hours old. This, however, is a fallacy. 



Fig. 23 

 APPEARANCES OF THE NUCLEUS 



(1) Vesicular vegetative nucleus in a Gram-negative coccus. Methylene-blue-eosin. 



(2) False appearance of vesicular nucleus in multicellular coccus, .\ctually the nuclear 

 material of several cells is condensed centrally. Methyl-violet-nigrosin. 



(3) As (1) in Sarcina sp. 



(4), (5) Nuclear bodies in Mycobacterium lacticola and in Xdcui'diii sp. These appear 

 spherical, but this may be because they are too small to be resolved. 



(6) Appearance of vesicular nucleus in acid-hydrolysed cells of Aerobacter sp., probably 

 due"Ho laking of the stain. 



All plates at / 3000. 



