E. Hindle 
155 
In its staining reactions Rickettsia shows a very close resemblance to 
Spirochaetes, as the cytoplasm seems to contain a relatively small amount of 
chromatinic substance which is diffused evenly through it. For demonstration 
purposes Giemsa or Leishman is by far the best stain to employ, as the colour¬ 
ing matter is precipitated round the surface of the organism, and therefore it 
appears somewhat larger and has a fairly distinct outline. 
As noticed by previous observers, Rickettsia from the gut of the louse 
presents the appearance of round, oval, or diploid bodies, often occurring in 
clumps which are obviously the result of continued division without separation 
of the individuals. The parasite stains a purplish-blue colour with Giemsa and 
its dimensions may be estimated as about 0-3 p, in diameter by 0*3-O5/i in 
length. When dividing, the chromatinic material is concentrated at the oppo¬ 
site poles of the cell, thus giving the appearance of two darker stained bodies 
united by a somewhat lightly stained substance. Eventually the two halves 
separate, but as division seems to proceed rapidly these so-called diploid forms 
are usually present and form a characteristic feature. 
The examination of sections and wet films has not resulted in finding any 
structure in Rickettsia which cannot be observed just as well in dried films, 
and as the latter are much simpler to prepare their use is recommended for 
this organism. 
With regard to the occurrence of R. quintana in the alimentary canal of 
the louse the parasites are confined to the lumen of the gut and I have not 
observed their entry into the epithelial cells lining the alimentary canal or 
into any other parts of the body. The organisms seem to become attached to 
the surface of the epithelial cells of the midgut, and as they multiply gradually 
fill the lumen of the gut and extend backwards down the intestine, and eventu¬ 
ally are voided together with the faeces. 
The results of these observations confirm those obtained by Arkwright, 
Bacot and Duncan (1919) in their important series of experiments on the 
association of Rickettsia with trench fever. With regard to the systematic 
position of the organisms themselves, there is nothing to distinguish them 
from bacteria except their minute size. 
The multiplication in the mid-gut and subsequent extension into other 
parts of the alimentary canal resembles that of Bacillus pestis in the gut of 
the rat flea. The insect host, in the case of R. quintana , merely furnishes a 
living culture tube in which the organisms grow and multiply without passing 
through any cyclical changes comparable with those that take place in the 
case of protozoa. The negative period of four days which intervenes between 
the ingestion of blood containing Rickettsia and the faeces of the louse be¬ 
coming infective, is probably merely the result of the Rickettsia not being 
present in sufficient numbers to produce any infection, for it is well known in 
the case of other diseases that the inoculation of less than a minimum number 
of organisms does not produce any noticeable infection. 
