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PERIODICITY 



207 



within their sheaths, presenting an appearance similar to that 

 shewn in Fig. 54. 



These are the filariae that one encounters on examining 

 the blood of a filarious patient, and as will be seen they are 

 merely immature forms and, consequently, it is very difficult 

 to classify them, as the various species are distinguished 

 mainly by the characters of the adults. These embryos are 

 often termed " microfilariae " in order to distinguish them from 

 the mature " filariae," but it should be remembered that such 

 a term cannot be used in a generic sense. 



Fig. 54. Microfilariae of F. bancrofti emerging from the uterus of the parent 

 filaria, uncoiling in their chorionic envelopes. {After Bahr, from 

 Filariasis and Elephantiasis in Fiji.) 



Periodicity. The number of filariae in the peripheral cir- 

 culation presents remarkable periodic variations, a pheno- 

 menon which was first noticed by Manson. In the case of 

 Filaria bancrofti, this author found that, in China, the parasites 

 were present in the peripheral circulation during the night, 

 but disappeared during the day ; and he proposed the specific 

 name nocturna in order to express this habit. Under normal 

 conditions of health and habit, during the day the parasite is 



