90 THOMPSON YATES LABORATORIES REPORT 
Periodicity. It was this phenomenon, and this alone, which led Manson to 
regard F. nocturna and F. diurna as distinct species. And certainly, in the limited 
condition of the knowledge of the subject, it was a very natural conclusion, one 
large set of cases which had been examined, shewing a characteristic periodicity with 
a maximum number of embryos present in peripheral blood at midnight, and a 
smaller set presenting the reverse conditions, a maximum number at midday. The 
departure from this interesting regularity to be first noted, was recorded by Thorpe 
in the Tonga Islands where a large percentage of the adults shewed symptoms of 
elephantiasis, and where an examination of a large number of natives proved the 
presence of embryos in their peripheral blood both during the day and during the 
night in approximately equal numbers, and moreover shewed that the embryos were 
present throughout the whole of the day. 
We have already given details of several cases illustrative of the same con- 
ditions (table VII), and furthermore we have shewn (tables VIII and X) that cases of 
filarial infection occur in whom the hour at which the maximum number of embryos 
is present in peripheral blood is not mid-day and midnight, but may be any 
other hour — 3, 6, or 9 a.m. or p.m. And besides we have shewn that ' pure ' cases 
of F. diurna and F, nocturna are considerably less frequent in West Africa than these 
irregular cases. 
The definitive hosts. Thorpe, probably bearing in his mind the classical 
experiment of Mackenzie, and the repetition of that experiment in another case by 
Man son, by which it was proved that by a change in the habits of a case of F. 
nocturna, the periodicity of the embryos could be completely inverted, becoming thus 
similar to that of F. diurna, explained the peculiar phenomenon of the occurrence of 
the embryos in the blood of the natives of the Friendly Islands by the habits of the 
natives, which he thus describes from Mariner's classical account of the Tonga 
Islands : 
' The natives employ themselves in conversation not only at any time during 
the day but also at night. If one wakens, and is not disposed to sleep again, he 
wakens his neighbour to have some talk. By and by, perhaps they are all aroused, 
and join in the conversation. It sometimes happens that the chief has ordered his 
cooks in the evening to bake a pig or some fish and bring it hot in the middle of the 
night with some yams. In this case the torches are lighted, and they all get up to 
eat their share, after which they retire to their mats ; the torches are put out, some 
go to sleep, and others talk perhaps till daylight.' 
Similar habits are in practice among the natives of the whole of West Africa, 
but to a larger extent and on a larger scale. We were often told by natives from 
different parts of the Coast that it is common practice in the respective countries to 
which they belong, to sing and dance the whole night through, especially on moon- 
light nights. In fact we have ourselves heard the midnight orgies in the native 
