C. F. Robinow 



423 



top corner the cells in this picture were dormant at 

 the time of fixation. 

 Fig. 24. The same culture as in fig. 25, incubated for 

 90 min., fixed and stained in the same way. a, a cell 

 of the slender rod type which has not yet begun to 

 develop into a large two-cell bacterium, b, a two-cell 

 bacterium, c, a slightly constricted four-cell bac- 

 terium, e, dividing four-cell bacteria. 



Figs. 25, 27 are successive stages in the development 

 of B. mycoides beginning with spores near the end of 

 the germination period. 



Fig. 26 should be compared with the corresponding 

 stages a, b, d in the development of Bact. coli in PI. 5, 

 figs. 1, 2. 



Fig. 26. Demonstrates a clear median cell boundary. 



The three bacteria in fig. 27 correspond to those in 

 figs. 8 and 9 of Bad. coli on PI. 5. At a a strand of 

 chromatinic matter is seen extending between sepa- 

 rating chromatinic structures. 



Plate 8 

 B. mesentericus, figs. 28, 29 

 B. megatherium, figs. 30-34 

 Bad. coli, figs. 35-37 

 No nuclear structures are shown in any of the photo- 

 graphs on this plate. The photographs of B. megatherium 

 are from water-mounted preparations, the rest from 

 balsam mounts. 



Figs. 28, 29. B. mesentericus from spores incubated for 

 2 hr. and 2| hr. respectively, a in fig. 28 marks a 

 single-cell stage, all the remaining bacilli in both 

 pictures are composite in nature. Note the dark 

 staining of the tips and of the places where the bac- 

 teria will eventually divide by constriction. The 2- 

 and 4-cell bacilli in fig. 28 are all 'resting', the two 

 long diagonally placed filaments in fig. 9 show narrow 

 median constrictions. Each consists of at least four 

 cells. Compare with PI. 7, fig. 24 of Bact. coli and 

 note similarity in structure. 

 Figs. 30-34. From three different preparations of 

 B. megatherium, treated with boiling 1-3% NaOH as 

 described under method 5. The stain is 0-5 % crystal- 

 violet and the preparations were photographed 

 mounted in water. 

 Fig. 30. Plasmolysis and differentiation of cytoplasm 

 and cell wall are imperfect but the composite structure 

 of all three bacilli is nevertheless recognizable, a, the 



cytoplasm of the left half is incompletely, that of the 

 right half completely divided by a transverse cyto- 

 plasmic partition not yet strengthened by cell-wall 

 material. The specimen might be described as a 3-4 

 cell bacillus. Had the plasmolysing treatment been 

 more effective, it is probable that the internal con- 

 figuration would have resembled that of the right half 

 of the large dividing specimen in fig. 33. b, note the 

 retraction of deeply stained cytoplasm from the cell 

 wall in the plane of constriction and from the hori- 

 zontal part of the adjoining region (under side) of the 

 third cell from the left. There has also been a concave 

 retraction of the cytoplasm in cell no. 4 at the right 

 extremity of the bacillus. (Compare this specimen 

 with the large, darkly stained bacterium to the left 

 of the centre in PI. 7, fig. 24d.) There has been some 

 retraction of the cytoplasm from the cell wall at both 

 poles of the third bacillus (c). 



Fig. 31. The two cells on either side of the future plane 

 of constriction in this bacillus have retracted in two 

 continuous portions from the cell wall where the forma- 

 tion of transverse septa, destined to cut into the cyto- 

 plasm, can be seen to have begun. In the right-hand 

 top corner is a discarded spore case. 



Fig. 32. All four component cells of this young bacillus 

 have retracted independently from the outer cell wall. 

 The third cell from the left has also retracted from 

 the transverse partition separating it from the fourth 

 cell. The left end of the bacillus is still contained in 

 the spore case. 



Figs. 33, 34. All-round retraction of the cytoplasm from 

 the cell wall in the bacillus on the right has provided 

 a clear view of the medium transverse partition. 

 Further examples are seen in the right-hand top corner 

 and near the left margin of fig. 34. 



Figs. 35-37. Bact. coli, incubated for 3 hr. on an agar 

 plate at 37° C. followed by 20 hr. 2-4° C. Bourn fixation 

 through the agar, Giemsa's stain. Mounted in Canada 

 balsam. 



Fig. 35. Retraction of the cytoplasm from a median 

 septum. Probably comparable to the condition of the 

 upper half of the right-hand bacillus in the adjoining 

 fig. 34. 



Figs. 36, 37. In both figures the bacterium marked a is 

 composed of one deeply stained and one lightly stained 

 half. Demarcation of the two is particularly clear in 

 fig. 36. The faint staining is due to disintegration of 

 the cytoplasm in this half of the bacterium. Explana- 

 tion in the text. 



(MS. received for publication 17. ix. 43 -Ed.) 



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