MORPHOLOGY 141 



The higher percentage (28.3 per cent) of positive smears as 

 compared with 21.5 per cent positive sections obtained by 

 the examination of 705 lice from experimental boxes indicates 

 that it is rather more difficult to find Rickettsia prawazeki by 

 means of sections than by smears. 



As a result of the combined study of smears and sections 

 from infected lice, we believe that Rickettsia prawazeki under- 

 goes a developmental cycle in the louse in which the first 

 stages are the thread or filamentous and bacillary forms. The 

 minute paired rickettsia bodies or coccoid forms probably 

 represent the stage of most active multiplication. The deep 

 stained lanceolate form is possibly the latest stage of develop- 

 ment in the louse and a quiescent stage of slightly greater 

 resistance to external agents. 



3. THE RECOGNITION OF RICKETTSIA 



In smears of lice and of tissues occur granules consisting of 

 cellular debris and occasionally bacteria which cannot be 

 definitely distinguished from isolated rickettsia. When rickett- 

 sia do occur in lice they are usually present in very large num- 

 bers, consequently, in practice rickettsia have never been 

 definitely accepted as being present unless they occurred in 

 considerable numbers and unless there was a wide range of 

 variation in their size and form. Because of their pleomor- 

 phism, and because of uncharacteristic shapes, it is impossible 

 to definitely identify any isolated organisms, or even group of 

 granules, as rickettsia. In some of the Polish control lice, 

 granules were seen which might have been rickettsia but which 

 it was quite impossible to identify as such. 



Because it is usually impossible to decide the origin of 

 isolated structures seen in smears, the study of smears is often 

 inconclusive and is much less satisfactory than the examina- 

 tion of sections. Among the heterogeneous and confusing 

 debris seen in smears are hemoglobin crystals from the gut of 

 lice. Sometimes these occur in very small rods and, if they 

 take the stain, much resemble bacteria. Confusion is sometimes 

 caused by granular substances representing coagulated and 



