8. Yokogawa i(53 
layers, the inner one of which has not been observed before. The outer one 
probably represents an incomplete moult, and is much wider than the inner 
one. The inner cuticula is thin and has only transverse striations and not 
the longitudinal ridges which are so prominent on the outer cuticula. These 
two layers of cuticula are fused at the anterior tip, at the posterior limit of 
the cephalic inflation, at the anus, vulva and the tail of the female and over 
the whole of the bursa of the male. 
2. The oviduct of the adult female was not distinguished in my first 
study. However, by the study of the life-history it is made clear, measuring 
about 0-07 to 0*09 mm. in length. 
3. The structure of the second part of the ovijector (pars ejectrix of the 
ovijector) was not clearly understood before. Now I understand its structure 
by the- study of its post-embryonal development. 
VI. SUMMARY. 
1. Hehgmosomum muris proved to be very favourable material for the 
study of nematode development, since it will develop perfectly normally in 
culture rats, infection is easily carried out and since sexual maturity is reached 
in 7-10 days after infection. 
2. The post-embryonal development of H. muris is divided into five stages, 
two free and three parasitic, with three moults. There is only one moult during 
free life, the second and third stages being separated by change of habitat 
brought about by entrance into the host. Sexual maturity is attained soon 
after the completion of the third moult. The mature worm has two cuticular 
layers, the outer of which is separated by a space from the inner. This outer 
cuticula is probably the beginning of a fourth moult which is never com¬ 
pleted. 
3. Under favourable conditions the eggs hatch in about 20 to 24 hours 
after being passed with the faeces. 
4. The first two stages of post-embryonal development, which are passed 
in free life, are separated by a relatively long moult during which the larva 
changes from the rhabditiform type to the filariform type. During this period 
there is a rapid division of the cells lining the intestine, which frees masses 
of these cells into the lumen and leaves the intestine of the filariform larva 
lined with flattened cells. 
5. The infective stage is not enclosed in a sheath and tends to work its 
way out of the culture onto the glass or along the edges of the filter paper. 
At this stage it is impossible to distinguish the sexes. 
6. Infection of the rat can be accomplished both by way of the mouth 
or through the skin although the latter method is by far the most effective. 
The larvae reach the lungs about 14 to 20 hours after penetration through 
the skm. They remain in the lungs until about 35 to 65 hours after infection. 
The majority of them reach the intestine 50 to 65 hours after infection, 
although in a few they were found as early as 45 hours. 
11—2 
