166 
Development of Hook-worms 
conditions it is difficult to say. Abnormal stools are not a usual or 
even a common accompaniment of hook-worm infection. There is in 
fact no characteristic faecal condition in hook-worm disease, and the 
changes above noted must be regarded as ha\'ing their origin in some 
intercurrent condition. 
The subject is one that would require prolonged study and it is 
doubtful if the results would have much practical bearing on hook-worm 
disease. 
Turning now to another possible abnormal condition, there is the 
nature and quality of the soil or land over which the larvae, after their 
escape from the faecal matter, are scattered. It has already been 
stated that fully developed larvae can exist for many months in ordinary 
tap water or rain water. They can also be kept alive for long periods 
in distilled water and in normal saline solution (0-75 %). Under natural 
circumstances the medium in which the larvae live is rain water with 
the addition of certain salts and other elements taken up from the 
ground. These will, in every case, depend on the nature of the ground. 
Only a limited number of observations have been made on this 
important matter, either under natural conditions or in the laboratory. 
It has been mentioned in an earlier part of this paper that common 
salt (sodium chloride) in the strength of about 2 or 3 % has been observed 
to have a deleterious effect upon the eggs and larvae of hook-worms. 
That this is not invariably the case will be shown later. This is 
practically the only salt on which observation or experiment has hitherto 
been made. 
A few observations have been made on the effect of acid (sulphuric) 
and it has been shown that very weak solutions will kill both eggs and 
larvae. 
There remains to be considered abnormal conditions of food supply, 
air supply, temperature and moisture. 
These matters have been discussed very fuUy and lucidly by Looss 
and by Boycott and little more need be done here than to combine and 
summarise their results. Several other workers have made detached 
observations on these lines, but as their work has been carefully 
examined and criticised by Looss it is unnecessary to refer to them 
here. It may be added that I have personally tested most of the 
more important observations and have been able to confirm them 
except in cases which will be noted. 
A proper and adequate supply of food is necessary for the growth 
of the larvae. Looss came to the conclusion that faeces derived from 
