S. Yokogawa 139 
number of larvae on the edges of the filter paper and these larvae will be more 
active. 
On the contrary, if the larvae are exposed to direct sunlight, they soon 
become quiet and in a short time will be found to be dead. Further the eggs 
will not develop and hatch when the cultures are kept in direct sunlight. This 
relation to light is very interesting when we consider that the normal environ¬ 
ment for the development of the larvae, and the place where they would 
most probably infect their hosts, would be in the dark burrows of the rats. 
(c) Moisture. 
I did not carry out any definite series of experiments to determine the 
most favourable degree of moisture or the resistance of the larvae to desicca¬ 
tion. It is evident, however, that some moisture is necessary for the develop¬ 
ment of the eggs and larvae of H. muris. I was able to observe that the eggs 
and larvae develop well under the condition of moisture, in which the filter 
paper used for culturing was kept about as moist as the faeces are at the time 
of defecation. If the faeces are soaked with water, the development often 
failed. On the contrary, if the cultures are not kept quite moist the larvae 
do not develop well. 
The resistance of the larvae to desiccation varies according to the degree 
of their development. The mature larvae are more resistant than the other 
stages. They were found alive for a month or more on the walls of the covered 
containers in which the cultures were kept. They were also alive on a glass 
slide which was exposed in the air for more than two days. The young larvae 
were killed in dried cultures and on a glass slide in several hours. 
(d) Tem/perature. 
I have no definite experimental data to show what temperature is most 
suitable for the development of the eggs and larvae. My experience with 
culturing showed that the development was more rapid during the hot summer 
period than during the spring and autumn when the temperature was lower. 
III. METHOD OF INFECTION AND MIGRATION IN THE HOST. 
After I had learned to culture infective larvae in quantity I tried two series 
of experiments on white rats to determine the method of infection with 
H. muris. In one series of experiments the mature larvae were fed to the rats 
and in the other they were brought into contact with their skin under conditions 
favourable for penetration. 
1. Infection through the Mouth. 
In this experiment seven tame rats were given great numbers of the mature 
larvae of H. muris on bread and watched carefully to make sure that they 
actually ingested the larvae. All of the rats were dissected at varying periods 
after the feedings and were found to contain only small numbers of the 
parasite, as summarized in Table III. 
