S. Yokogawa 
155 
genital primordium (Fig. 17). At the beginning of the first parasitic stage 
it is represented only as an extremely fine strand of tissue. Then this tissue 
strand becomes a little larger, taking the shape of a small tube with one spindle- 
shaped cell in each side (Fig. 17). The length of this narrow central region 
varies greatly in different specimens. This region at the beginning of the 
division of the genital primordium is short, having a length of 0-02 to 0-04 mm. 
At the beginning of the second parasitic stage it has become much extended, 
measuring 0-06 to 0-0115 mm. in length, and then it shortens again, showing 
a tubular structure. Toward the end of the second parasitic stage it becomes 
much broader, with two cuboid cells on each side of its wall, where previously 
there was only one spindle-shaped cell (Fig. 20, vd, Plate X). These cuboidal 
cells are close together and are separated from the adjoining cells both ante¬ 
riorly and posteriorly by definite constrictions (Fig. 20, vd). These constrictions 
become less clearly defined with later development. The fully developed vas 
deferens is narrower than the other regions of the male reproductive organs, 
having a length of about 0-02 to 0-03 mm. and is situated on the dorsal side 
of the intestine near the middle of the body. 
Development of the seminal vesicle. The seminal vesicle develops from the 
anterior region of the posterior part of the genital primordium. At the begin¬ 
ning of the second parasitic stage it is not clearly differentiated. Toward the 
end of the second parasitic stage and especially during the last moult the 
region just behind the vas deferens has a wall consisting of clear cuboidal cells, 
while the part just beyond is lined with high columnar cells containing fine 
granules. The seminal vesicle (Fig. 20, sv) is produced from the region lined 
with the clear cuboidal cells and becomes a large tube with a wide lumen. 
The cells of the wall of this region decrease in thickness with the increasing 
of the lumen, and finally a thin membrane is produced in which it is difficult 
to make out the individual cells. At this time the lumen of the seminal vesicle 
contains a few sperms. The seminal vesicle of the adult worm is situated at the 
dorsal side of the intestine near the middle part of the body, and has a length 
of about 0-1 to 0-13 mm., and is filled with spermatozoa. 
The development of the cement gland. With the formation of the seminal 
vesicle the region which is to form the cement gland becomes more clearly 
defined. This region is lined with high columnar cells which at first are all 
alike. Each of these cells contains a round nucleus and many fine granules. 
When the internal setf-organs are almost developed the cement gland becomes 
differentiated into two parts by the appearance of the cells of its wall (Fig. 20, 
cmg, Plate X). The cells of its posterior half become darker than those of its 
anterior half and contain a great amount of granules. In the adult worm, this 
gland usually is situated where the reproductive tube crosses the intestine, 
while in the earlier stages it is found farther back, on the ventral side of the 
intestine. 
The development of the ejaculatory duct. The ejaculatory duct is produced 
from the posterior part of the genital primordium. The cells of its wall early 
