34 
Rickettsia-^’&e Parasite of Cimex 
Developmental cycle of the parasite. 
The scheme set out below is merely tentative, all that is claimed for it is 
that it is a reasonable explanation of the observed facts and the excuse is 
offered that by linking up these facts a clearer picture can be conveyed than 
by the mere recital of the detached details. In the absence of any success in 
cultivation apart from the host, it is of course only possible to assume a 
connection between some of the forms in the order in which they are stated. 
Starting with the eggs within the ovary, it seems most probable (a) that 
they become infected at the time of fertilisation with the Rickettsia form 
(Plate II, fig. 7), ( b ) that simple multiplication is followed by some of 
the first generation developing through a bacillary stage into threads while 
others continue simple multiplication pari passu with the presentation of 
suitable conditions due to the development of the embryo, (c) that owing to 
the massive granule infection of the egg material at this stage, cells of 
practically every organ of the growing embryo tend to become involved, but 
only in a certain number are the conditions necessary for intracellular 
multiplication afforded. In these, clusters of the minute Rickettsia forms 
develop, rapidly changing in favourable situations into the red-staining 
lanceolate forms through the development of an outer covering or envelope. 
These enlarge in due course into the long bacillary forms which in their 
turn release the darkly staining minute Rickettsia bodies. The large cells of 
the Malpighian tubules and their free unencumbered position allow of a much 
more massive infection than other situations; it is also possible that they 
afford more stable conditions during the moulting periods and bring the 
organisms into close proximity to the sexual glands when these are developed, 
thus increasing the chances of the latter becoming infected. In the course of 
the very numerous dissections which it has been necessary to make in the 
progress of this work, it has been noticed how frequently the enlarged cells 
occur in the distal third of the tubules. 
Suggested name for the parasite. 
The organisms known as Rickettsia are at present very incompletely 
described and only recognisable by their more superficial characters, such as 
morphology and localisation. 
On such evidence it is not possible to decide with any certainty how near 
or remote the phylogenetic relationship of the members of the group to one 
another may really be, and the present writers therefore consider that 
Rickettsia should not be accepted as a generic name in the strict sense of the 
term to cover all the organisms referred to in their opening paragraphs. The 
smaller group comprising only those concerned with the three mammalian 
(human) diseases typhus, trench, and Rocky Mountain spotted fevers, would 
seem to form a more convenient unit. 
Nevertheless, since the Rocky Mountain spotted fever organism has been 
