150 
Nematodirus filicollis Rud. 
The genital rudiment also showed a considerable increase not only 
in actual size but also in the number of its constituent cells (Text 
fig. 3). 
Mode of Infection of Sheep. 
It was unfortunately found impossible for various reasons to perform 
infection experiments on sheep, I am therefore not in a position to 
produce direct proofs that these animals become infected by swallowing 
the sheathed larvae of Nematodirus filicollis. The circumstantial 
evidence in favour of such direct infection is, however, very strong: 
in the first place, as already pointed out, in their structure, general 
behaviour and especially in their migratory instincts, these larvae are 
precisely similar to those of forms known to infect warm-blooded 
animals directly; the conditions under which the second ecdysis is 
completed also greatly favour this view; whilst lastly, as will be shown 
in the next paragraph, examination of the intestinal contents of sheep 
harbouring adult Nematodirus revealed practically all stages between 
mature larvae and adult individuals, the youngest of these intermediate 
stages showing but little advance in structure over the “ exsheathed” 
larvae just described. 
Whilst the larvae in the free condition are probably the more usual 
source of infection it must not be forgotten that these are no doubt 
infective whilst still within the egg-shell, being already ensheathed 
before hatching. That the mature eggs are a possible source of infection 
was indicated by Railliet and Henry (1912) when in their definition of 
the genus they stated “l’embryon.. .est apte a rentrer directement dans 
l’orgauisme sans phase de liberte dans le milieu exterieur.” 
Young Stages in the Sheep. 
The alimentary tracts of infected sheep and lambs were carefully 
examined for young parasites, and a number of interesting early stages 
of Nematodirus filicollis were obtained; the youngest of these were 
only little more advanced than the “ exsheathed” larvae described in a 
preceding paragraph. 
The youngest stage was found in a lamb killed in March, 1914, it 
measured only T2mm. in length and except for its greater relative breadth 
and for an increase in the number of intestinal and genital cells is very 
similar to a mature larva just after ecdysis (Text-fig. 4). The body has 
a maximum thickness of 40/x and is approximately cylindrical in shape, 
tapering however at both extremities. The truncated head measures 
