W. Nicoll 
171 
faeces was put and the vessels again weighed. The weight of faeces 
used in each experiment was thus known. In the earlier experiments 
the vessels used were small stout bottles with wide necks and loosely 
fitting corks, with wooden tops. In the later experiments, however, 
small glass pots with heavy glass tops were used. The entrance of 
air was ensured by introducing a small paper plug between the pot 
and the cover. 
The various liquids and solids used were added in measured 
quantities so that an approximate idea could be obtained of the amount 
of material required. These substances were applied in the manner 
specified in the detailed account which follows. 
At intervals of 24 hours small portions of the mixtures were removed 
and examined to determine the condition of the eggs. Fragments were 
also thoroughly and repeatedly washed with water and the residue 
spread out in small Petri dishes. These were examined at regular 
intervals, usually 24 hours, to ascertain the state of development of 
the larvae. 
In most cases the experiments were continued until all the eggs 
were apparently dead-and no larvae could be reared from them. 
At this point it may be well .to discuss the various methods of rearing 
hook-worm larvae. 
The natural and, perhaps, ideal conditions for the development of 
hook-worm larvae have already been mentioned in the brief outline of 
the larval life-history, but it is not often easy or even desirable to 
reproduce these conditions in Jhe laboratory. The nearest approach 
to this may be obtained by exposing the faeces in an open Petri dish 
and drenching them with water every 24 hours. With each drenching 
a certain number of eggs and larvae are washed out and these can be 
reared in separate dishes. 
In practice this does not prove a particularly efficient method of 
rearing larvae. One objection to it is that the faeces dry up within 
24 hours. That can be remedied by drenching every 12 hours or 8 hours. 
Another objection is that flies, especially house-flies, deposit their eggs 
on the faeces and the resulting larvae rapidly eat up the material. 
At the same time it must be noted that these fly larvae in their wanderings 
distribute the faeces over a wide area and in a thin layer, a circumstance 
which is particularly favourable for the development of hook-worm 
larvae. To what extent this may aid the dissemination of hook-worms 
under natural conditions remains to be investigated. In the present 
series of observations, however, the majority of experiments to which 
Parasitology ix 
12 
