lyS SULPHUR BACTERIA 



undergoes a hardening process, is characteristic of yeast cells 

 under certain unfavourable conditions. Former investigators 

 of Beggiatoa alba have not distinguished with sufficient clear- 

 ness between sheath-walls and cell membrane. To sum up, 

 the cell membrane is a living, peripheral, plasmatic covering, 

 whereas the sheath is a non-living transformation of the latter. 

 Sulphur globules of an oily consistency and high refractive 

 power arc found throughout the cytoplasm. The number of 

 globules varies, and they may even be entirely absent. The 

 variation in their number is evidently connected with metabolic 

 changes, and their absence from the filaments usually indicates 

 unfavourable conditions. They vary considerably in size, 

 some being mere points whilst at the other extreme globules 

 of 2/A in diameter may be found. They present a very 

 characteristic appearance ; a thick black ring sharply defined 

 appears to enclose a clear interior (Figs. 6a, 6g). Each gives 

 an appearance under the microscope comparable to a thick 

 ring of black ink drawn on white paper. It is highly probable 

 that the development of a sulphur droplet is preceded by the 

 formation of a vacuole in the cytoplasm into which secretion 

 takes place. Arthur Meyer and the author (Ellis (i)) have 

 shown that the development of the bacterial spore takes 

 place by secretion into a previously prepared vacuole. The 

 development of the sulphur globule appears to follow similar 

 lines. In the investigation into the intimate structure of the 

 cell of Thioporphyra volutans, it is shown that the sulphur 

 globule possesses a centrum of organic matter. 



Tests for Sulphur. — Sulphur globules in plant cells have a 

 characteristic appearance. See Figs. 27 {a and c), 31, 32. The 

 centrum is hyaline. When the cells are treated with aceto- 

 carmine, the sulphur is dissolved and crystallizes outside the 

 cells in octohedra (see Figs, lyd, 34^) typical of this element. 

 When treated with a concentrated solution of sodium-nitro- 

 prusside, Na2[Fe(NO)(CN)5] . 2H2O, the rings of sulphur as- 

 sume a blood-red colour. 



There is no evidence of a nucleus, and no colouring matter 

 is developed. 



Motility. — The cause of movement is at present unknown. 



