CHAPTER V 



THE SIZE AND FORM OF THE CELLS OF 

 THE COLON BACILLUS 



In the studies of Bacillus megatherium immediately preceding, 

 the only measure of size considered was the length of the cells. 

 There were several reasons for this, the most important being that 

 the diameter of the cells is so small that by the technique used it 

 was impossible to make measurements of the thickness at all com- 

 parable in accuracy with the measurements of length. It was quite 

 apparent, however, that while there was a degree of correlation 

 between the length and diameter of the cells, the longer cells being 

 somewhat thicker, nevertheless the increase in thickness with in- 

 creasing length was by no means proportional; that is, the longer 

 cells were relatively much more slender, and it was assumed, there- 

 fore, that measurements of length alone would be sufficient to indi- 

 cate the character of the variations in size which were taking place. 

 It was found desirable, however, to make some observations on this 

 change in the relative proportions between length and thickness of 

 the cells, that is to obtain some sort of a quantitative expression 

 of the variations in the form of the cells which were taking place. 



This was done with a culture of Bacillus coli, using photomicro- 

 graphs projected so as to give a final magnification of 30,000 X- From 

 the tracings of the projected cells the area was measured, as well as 

 the length along the major axis of the cell and from these 

 two measurements the quantitative expression of the form of 

 the cell was obtained by dividing the area of the projected image by 

 the length squared. The quantity so obtained, which I have desig- 

 nated the area-length index, is an abstract measure of form, regard- 

 less of the size of the cell. It will have its highest value if the cell 

 from which the tracing is made is a sphere, that is the image being 

 circular. As the length increases in proportion to the width, this value 

 becomes progressively smaller. 



Such a measure suffices to express accurately the form of the 



87 



