S. Yokogawa 131 
tions of the free development of the larvae I finally became convinced that 
there was only one moult. Finally the finding of the double layers of the 
cuticula in the adult stage struck me as important in this connection. The 
separation of the outer cuticula at this stage goes on just as if the worm were 
preparing for a moult but it remains always attached. It is only a short time 
after the completion of the third moult that the double conditions of the 
cuticula develops, and the space between the two layers of cuticula gradually 
becomes larger. The outer cuticula of the female is much looser than that of 
the male and extends posteriorly, forming a peculiar sac (Yokogawa, 1920). 
The outer cuticula is thick and has the transverse striations limited to the 
prominent longitudinal ridges, while the inner one is thinner and has only 
the transverse striations. No other case of a double cuticula has been found 
recorded for nematodes. It is probable that this condition is a characteristic 
of the genus Heligmosomum, which has escaped notice in the other species 
that have been described. This condition undoubtedly represents an in¬ 
complete moult. Therefore it seems to me that we can safely conclude that in 
H. muris the condition is the same as in other nematodes but that the second 
moult is transferred into the parasitic life and the fourth is never completed. 
Looss (1911) divided the post-embryonal development of the nematodes 
into five periods which are separated by four moults. This method of division 
is commonly used. This division does not hold good for H. muris , since if 
in this form the stages were separated by the moults the development would 
include only four stages divided by three moults, the second stage including 
both a free and a parasitic stage and two distinct periods of development. 
In this case, it seems to be necessary to consider as a separate stage the time 
from the entrance into the final host to the second moult. This stage is then 
the third stage or the first parasitic stage and is separated from the second 
stage by the penetration of the larvae through the skin. 
The post-embryonal development of Heligmosomum muris is divided into 
five stages by three moults. The first two stages are passed outside the host, 
as free-living larvae, and are separated by a relatively long moulting period, 
in which there are many structural changes. There exists some difference of 
opinion in regard to exactly when the second larval stage in the free life of 
nematodes commences. Previous authors have described the second stage as 
© 
beginning after the larva has completed its first moult. From the biological 
point of view larvae are already in the second stage when the structural 
changes which go on during the moult have taken place. In the case of 
H. muris , the duration of the first moult is relatively long and remarkable 
structural changes occur during this period. Since it seems necessary for the 
purposes of description to establish a definite line of demarcation between 
the first and the second stages, I will here describe as a part of the first stage 
those changes which occur up to the completion of the first moult. 
The three last stages are passed inside the host. The third larval stage or 
the first parasitic stage js separated from the second larval stage not by 
9—2 
a 
