S. Yokogawa 
147 
since they are influenced by the development of the external sexual organs. 
In the male 15 hours after infection the rectum runs obliquely to the main 
axis of the body through a definite cell group. Later a space develops between 
the walls of the rectum and this cell group. This space (Fig. 9, rs, Plate IX) is 
clearly defined in specimens found 30 hours after infection, while it has not yet 
appeared in specimens within 24 hours after infection. The wall of the rectum 
consists of a thin membrane. The rectum of the larvae near the beginning 
of the first parasitic stage has a length of about 0-04 mm. while its length 
has increased to between 0*18 to 0-20 mm. by the end of the second parasitic 
stage. The space surrounding the rectum and also the rectal lumen becomes 
wider as the bursa develops. Toward the end of the second parasitic stage a 
cell situated dorsally to the rectal sphincter grows posteriorly along the dorsal 
side of the rectum (Fig. 15, pwc, Plate IX). At the same time a cell on the 
ventral side of the end of the ejaculatory duct extends posteriorly a short 
distance along the inside of the ventral wall of the body (Fig. 19, Plate X). 
These cells become thicker and from them is produced the wall of the cloaca of 
the adult male. With the completion of the third ecdysis, the membranous wall 
of the rectum breaks off from the thicker part and forms a peculiar cloaca. 
The structural changes of the rectum during the development of the female 
in the parasitic life are not as complicated as those of the male. In the beginning 
of the parasitic life, the tail of the female is usually curved vent-rally. The 
rectum runs obliquely through a cell group to the anus, and has a length of 
about 0*04 mm. (Fig. 22, Plate XI). The cell group surrounding the rectum 
grows until about the middle of the second parasitic stage and then gradually 
the cells seem to decrease in size and the number diminishes. At the beginning 
of the second parasitic stage, a cell which is situated just back of the anus 
projects very slightly (Figs. 27 and 30, pac, Plate XI). This projection is 
gradually seen to disappear as the cells of the tail degenerate. After the com¬ 
pletion of the third ecdysis the cells of the tail region lose their definite 
differentiation and nuclei are no longer visible (Fig. 31, Plate XI). The rectal 
lumen at this stage is lined with a definite chitinous membrane, and the rectum 
has a length of 0-02 to 0-03 mm. The tail of the adult female is very thin, curves 
ventrally like a hook and is surrounded by the peculiar sac formed from the 
loose folds of the outer cuticula. 
(d) The Excretory System. 
The excretory apparatus of H. muris was not worked out in detail but 
seemed to be similar to that system as described for other species of the family 
Strongylidae. I gained, however, some information on the development of 
this system during the course of my studies which will be included here. Only 
the excretory pore, the cervical glands, and the excretory vesicle were found. 
The excretory pore is situated on the ventral surface at a distance of 0-15 
to 0-25 mm. from the anterior end of the body, according to the stage of 
development. The excretory vesicle can be seen in the anterior region of the 
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