W. Nicoll 
168 
two, however, must be within certain limits. The most suitable tempera¬ 
ture is from 65°-85° F. (18°-30° C.). Below this range of temperature, 
development is very slow; above it, development is very rapid but 
the eggs and larvae are liable to die. 
With regard to the amount of moisture the conditions are somewhat 
more complicated. For the initial steps of development the most 
favourable condition is that in which the amount of moisture in the 
faecal material is maintained approximately constant, provided the air 
supply is not excluded. If the supply of air is deficient the development 
is retarded. An adequate food supply is necessary for the growth of 
the young larvae. 
Under suitable conditions then the egg develops rapidly. The two 
cells originally present in the egg divide again and again and eventually 
form a small larva. When fully formed within the egg shell this larva 
measures about 0-25 mm. in length. By the end of 24 hours it proceeds 
to make its escape from the egg shell. 
It .commences to feed and continues to grow in size for the next 
four or five days. At the end of the second day, when about 0-4 mm. 
in length, it sheds its skin and assumes a slightly different shape. At 
the end of the fourth or fifth day the skin again begins to be detached 
but is not actually shed. It remains as a sort of flexible sheath enclosing 
the larva, but not interfering with its movements. 
This constitutes the infective stage in the life-history of the worm, 
for it is only when they reach this stage that the larvae can infect 
another person. The larvae now measure about 0-6 mm. in length and 
are very actively motile. Unless they have been washed away 
previously from the faeces they now proceed to migrate to a more 
suitable locality, namely the nearest water pool or patch of moist 
ground. Provided even a trace of moisture be present they are capable 
of traversing considerable distances and may thus give rise to infection 
far from the place where the faeces were originally deposited. 
How far and how rapidly they may wander in this fashion is not 
definitely known, but judging from laboratory experiences their rate is 
probably not less than' five feet per hour, so that in the course of 
24 hours they may have wandered 40 yards. This means that an area 
of 5000 square yards might become infected within 24 hoiirs after 
the larvae started to migrate, i.e. within a week after the faeces were 
deposited. 
These larvae when they reach this final stage have ceased to feed, 
and they can remain alive for months and even years under suitable 
