68 
Insect Flagellates , etc, 
frequently reaches 100 per cent., such a supposition was quite natural, although 
direct evidence of any connection between the said flagellate and the sheep- 
trypanosome was lacking. 
Later, the morphology and life-history of Crithidia melophagia in the gut 
of the sheep-ked were described in detail by Swingle (1909) and Porter (1910), 
who arrived at the conclusion that it was a specific insect parasite. This 
seemed the more obvious to the authors since both they and Flu (1908) had 
discovered that infection in the “keds” was transmitted hereditarily, so that 
the necessity of a second host, the sheep, seemed to have been eliminated. 
Swingle (1911, 1911a) tested this view experimentally. He examined the 
blood of a great number of sheep; inoculated them with, and fed them on, the 
contents of sheep-keds’ guts infected with these flagellates, and put freshly 
hatched “keds 7 ’ on sheep previously washed in an antiseptic solution. All 
these experiments gave negative results, with the exception of two cases, in 
which young “keds” which showed no flagellates became infected with them 
after having lived on sheep for twelve days. This occurrence, however, the 
author explained as hereditary infection of the “keds.” 
Swingle’s experiments were conducted so exhaustively that the question 
regarding the presence of trypanosomes in sheep and their relation to the 
flagellates of the sheep-ked seemed to have been finally solved. 
There remained, however, several points which were not quite clear, and 
there were other methods by which the problem could be further investigated. 
The first point that required further elucidation was the alleged hereditary 
transmission of C. melophagia. Hereditary infection was described by Flu 
(1908), Swingle (1909) and Porter (1910) as the means by which infection is 
maintained in the sheep-keds. 
The second question to be solved was the possibility of cultivating the 
supposed trypanosomes from the sheep’s blood. The negative results of 
numerous blood examinations made by Pfeiffer (1905), who was the first to 
describe C. melophagia Flu (1908), Swingle (1911, 1911 a), and Porter (1910), 
and the scanty positive results obtained in the same way by Woodcock (1910) 
and Behn (1911, 1912) showed that, even if trypanosomes occur in the sheep’s 
blood, their number is so small that in most cases they escape detection. Now, 
it was shown by Crawley (1909, 1909 a, 1912) that it is possible to cultivate 
trypanosomes from the blood of cattle in which the ordinary examination of 
blood failed to reveal their presence. Stockman (1910) also discovered trypano¬ 
somes in British cattle which had hitherto escaped detection, and the observation 
has been confirmed by other authors. 
Having these points in view, and taking into account the general importance 
of this question, both from the theoretical and practical aspect, since the 
presence of even a slight trypanosome infection might, under favourable 
conditions, be capable of producing a serious disease in sheep, as in the case 
of other domestic animals similarly infected, I have endeavoured to test this 
question experimentally. 
