S. Yokogawa 
129 
II. THE FREE-LIVING LARVAL STAGES. 
1. Development of the Egg. 
(a) Morphology of the Egg. 
The eggs of Heligmosomum muris are ellipsoidal in shape and have a very 
thin shell. The average size is 0-05824 mm. by 0-03276 mm.; a minimum size 
is 0-0546 mm. by 0-0309 mm. and a maximum is 0-06188 mm. by 0-03458 mm. 
Segmentation of the egg begins within the uterus of the female. Eggs in the 
one-cell stage are found in the upper part of the uterus, while in the lower 
part of the uterus they are in the 4 to 16 cell stages. In fresh faeces, the 
embryo is found most frequently in the 16 to 20 blastomere stage, while 
eggs which have just been given off by the female are usually in the 4 to 16 
cell stages. Exceptionally, eggs may develop to the morula stage in the 
intestine of the host but they are usually found in earlier stages. 
(h) Formation and Hatching of the Embryo. 
The embryonic development of the larva and the hatching of the egg of 
Heligmosomum muris are very similar to these stages as described for other 
members of the family Strongylidae, therefore I will include only a brief 
outline of these phases of development. 
For the observation of the embryonic development, I placed eggs which 
had just escaped from the female in a liquid culture. These experiments were 
carried out in the beginning of the summer of 1920 in Baltimore, at temperatures 
favourable for the development. The period of embryonic development varies 
according to the temperature and the media in which the eggs are placed. The 
following notes are the summary of the observations from these experiments. 
In 4 to 6 hours after placing the eggs in the culture a large number of them 
were found in the morula stage and some even in the advanced morula stage. 
In 8 to 10 hours the majority of the eggs had reached the “tadpole” 
stage, and some already showed a few structural details. At this stage the 
anterior end is broader than the posterior and somewhat conical and the 
cylindrical, buccal-cavity is visible. The beginning of the oesophagus and the 
intestine can be seen as a strand of dark cells. 
After 12 to 15 hours many of the eggs were in the advanced “tadpole” 
stage, while others already contained the rhabditiform larva. In some cases 
the embryo had attained three times the length of the egg, and in a few of 
the eggs it was four times this length. The shape of the body of the larva is 
cylindrical, the head is conical, and the tail is long. The mouth cavity shows 
clearly in optical section as two longitudinal lines, each terminating posteriorly 
in a point. The rhabditiform oesophagus and the chyle intestine are also well- 
developed. At this period a few free larvae are occasionally found already 
hatched. 
In 17 to 20 hours about 20 per cent, of the eggs are found already hatched 
and the majority of eggs seem to hatch between 20 and 24 hours. 
Parasitology xiv 
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